Click the "Help me!" link in the top right hand corner (ish) of your screen, followed by "pop-up help files", followed by "faq" and then information on FENs can be found under no. 8.
The FEN for a given position can be found under 'game details'. It can be copied and pasted into Chessbase or some other chess software to allow you to set up the position and play around with prospective moves/lines.
There is no way of searching through all the games played on RHP for games with a certain FEN, but yep, you can do it for a database that you yourself have.
Any other questions then ask away!
Mark
🙂