You need to first download the packages rhdfs, rhbase, rmr2 and plyrmr and then install them. You can find the packages here https://github.com/RevolutionAnalytics/RHadoop/wiki. To install:
install.packages("<path>/rhdfs_1.0.8.tar.gz", repos=NULL, type="source")
install.packages("<path>/rmr2_2.2.2.tar.gz", repos=NULL, type="source")
install.packages("<path>plyrmr_0.2.0.tar.gz", repos=NULL, type="source")
install.packages("<path>/rhbase_1.2.0.tar.gz", repos=NULL, type="source")