Once you are in scrollback mode with Ctrl + A, ESC, you should be able to search backwards through the buffer as described in the manual.
So to search for the word "string", press Ctrl + A, ESC, enter? string and press Enter. It will take you back through the buffer to the word string. Pressing N will go to the next match (backwards) in the buffer.
To search forwards, the cursor should be anywhere but the end of the buffer, then use /string to search down.