Hey @Dipti, seems like you haven't enabled the puppetlabs repo. I imitated your problem and enabling this repo worked fine for me:
$ rpm -Uvh --oldpackage --replacepkgs http://yum.puppetlabs.com/el/6/products/i386/puppetlabs-release-6-11.noarch.rpm
Retrieving http://yum.puppetlabs.com/el/6/products/i386/puppetlabs-release-6-11.noarch.rpm
warning: /var/tmp/rpm-tmp.PpkX2S: Header V4 RSA/SHA1 Signature, key ID 4bd6ec30: NOKEY
Preparing... ########################################### [100%]
1:puppetlabs-release ########################################### [100%]
$ yum install puppet-dashboard -y
Loaded plugins: fastestmirror, security
Loading mirror speeds from cached hostfile
* base: mirror.econdc.com
* extras: mirrors.vooservers.com
* updates: mirror.econdc.com
puppetlabs-products | 2.5 kB 00:00
puppetlabs-products/primary_db | 138 kB 00:00
updates | 3.4 kB 00:00
updates/primary_db | 1.9 MB 00:00
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package puppet-dashboard.noarch 0:1.2.23-1.el6 will be installed
--> Processing Dependency: ruby(abi) = 1.8 for package: puppet-dashboard-1.2.23-1.el6.noarch
--> Processing Dependency: ruby > 1.8.7 for package: puppet-dashboard-1.2.23-1.el6.noarch
--> Processing Dependency: ruby-mysql for package: puppet-dashboard-1.2.23-1.el6.noarch
--> Processing Dependency: /usr/bin/ruby for package: puppet-dashboard-1.2.23-1.el6.noarch
--> Processing Dependency: rubygems for package: puppet-dashboard-1.2.23-1.el6.noarch
--> Processing Dependency: rubygem(rake) for package: puppet-dashboard-1.2.23-1.el6.noarch
--> Running transaction check
---> Package ruby.i686 0:1.8.7.374-3.el6_6 will be installed
---> Package ruby-libs.i686 0:1.8.7.374-3.el6_6 will be installed
--> Processing Dependency: libreadline.so.5 for package: ruby-libs-1.8.7.374-3.el6_6.i686
---> Package ruby-mysql.i686 0:2.8.2-1.el6 will be installed
---> Package rubygem-rake.noarch 0:0.8.7-2.1.el6 will be installed
---> Package rubygems.noarch 0:1.3.7-5.el6 will be installed
--> Processing Dependency: ruby-rdoc for package: rubygems-1.3.7-5.el6.noarch
--> Running transaction check
---> Package compat-readline5.i686 0:5.2-17.1.el6 will be installed
---> Package ruby-rdoc.i686 0:1.8.7.374-3.el6_6 will be installed
--> Processing Dependency: ruby-irb = 1.8.7.374-3.el6_6 for package: ruby-rdoc-1.8.7.374-3.el6_6.i686
--> Running transaction check
---> Package ruby-irb.i686 0:1.8.7.374-3.el6_6 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
=====================================================================================================================================================================================
Package Arch Version Repository Size
=====================================================================================================================================================================================
Installing:
puppet-dashboard noarch 1.2.23-1.el6 puppetlabs-products 4.3 M
Installing for dependencies:
compat-readline5 i686 5.2-17.1.el6 base 128 k
ruby i686 1.8.7.374-3.el6_6 updates 538 k
ruby-irb i686 1.8.7.374-3.el6_6 updates 317 k
ruby-libs i686 1.8.7.374-3.el6_6 updates 1.6 M
ruby-mysql i686 2.8.2-1.el6 puppetlabs-deps 45 k
ruby-rdoc i686 1.8.7.374-3.el6_6 updates 381 k
rubygem-rake noarch 0.8.7-2.1.el6 base 404 k
rubygems noarch 1.3.7-5.el6 base 207 k
Transaction Summary
=====================================================================================================================================================================================
Install 9 Package(s)
Total download size: 7.9 M
Installed size: 17 M
Downloading Packages:
(1/9): compat-readline5-5.2-17.1.el6.i686.rpm | 128 kB 00:00
(2/9): puppet-dashboard-1.2.23-1.el6.noarch.rpm | 4.3 MB 00:02
(3/9): ruby-1.8.7.374-3.el6_6.i686.rpm | 538 kB 00:00
(4/9): ruby-irb-1.8.7.374-3.el6_6.i686.rpm | 317 kB 00:00
(5/9): ruby-libs-1.8.7.374-3.el6_6.i686.rpm | 1.6 MB 00:00
(6/9): ruby-mysql-2.8.2-1.el6.i686.rpm | 45 kB 00:00
(7/9): ruby-rdoc-1.8.7.374-3.el6_6.i686.rpm | 381 kB 00:00
(8/9): rubygem-rake-0.8.7-2.1.el6.noarch.rpm | 404 kB 00:00
(9/9): rubygems-1.3.7-5.el6.noarch.rpm | 207 kB 00:00
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Total 2.2 MB/s | 7.9 MB 00:03
warning: rpmts_HdrFromFdno: Header V4 RSA/SHA1 Signature, key ID 4bd6ec30: NOKEY
Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-puppetlabs
Importing GPG key 0x4BD6EC30:
Userid : Puppet Labs Release Key (Puppet Labs Release Key) <info@puppetlabs.com>
Package: puppetlabs-release-6-11.noarch (installed)
From : /etc/pki/rpm-gpg/RPM-GPG-KEY-puppetlabs
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
Warning: RPMDB altered outside of yum.
Installing : compat-readline5-5.2-17.1.el6.i686 1/9
Installing : ruby-libs-1.8.7.374-3.el6_6.i686 2/9
Installing : ruby-1.8.7.374-3.el6_6.i686 3/9
Installing : ruby-irb-1.8.7.374-3.el6_6.i686 4/9
Installing : ruby-rdoc-1.8.7.374-3.el6_6.i686 5/9
Installing : rubygems-1.3.7-5.el6.noarch 6/9
Installing : rubygem-rake-0.8.7-2.1.el6.noarch 7/9
Installing : ruby-mysql-2.8.2-1.el6.i686 8/9
Installing : puppet-dashboard-1.2.23-1.el6.noarch 9/9
Verifying : rubygem-rake-0.8.7-2.1.el6.noarch 1/9
Verifying : puppet-dashboard-1.2.23-1.el6.noarch 2/9
Verifying : ruby-rdoc-1.8.7.374-3.el6_6.i686 3/9
Verifying : ruby-mysql-2.8.2-1.el6.i686 4/9
Verifying : ruby-libs-1.8.7.374-3.el6_6.i686 5/9
Verifying : rubygems-1.3.7-5.el6.noarch 6/9
Verifying : ruby-irb-1.8.7.374-3.el6_6.i686 7/9
Verifying : ruby-1.8.7.374-3.el6_6.i686 8/9
Verifying : compat-readline5-5.2-17.1.el6.i686 9/9
Installed:
puppet-dashboard.noarch 0:1.2.23-1.el6
Dependency Installed:
compat-readline5.i686 0:5.2-17.1.el6 ruby.i686 0:1.8.7.374-3.el6_6 ruby-irb.i686 0:1.8.7.374-3.el6_6 ruby-libs.i686 0:1.8.7.374-3.el6_6 ruby-mysql.i686 0:2.8.2-1.el6
ruby-rdoc.i686 0:1.8.7.374-3.el6_6 rubygem-rake.noarch 0:0.8.7-2.1.el6 rubygems.noarch 0:1.3.7-5.el6
Complete!