You basically just have to wade through all the upcoming tournaments. Probably won't be able to find a seniors tournament but lots of under 1500 so I assume there must occasionally be one for <1200. Roll your cursor over to home and click on tournaments. Now click on "view all upcoming tournaments" in the turn-based section. Have fun! Oh and I hope you won't be offended by this but there is at least one senior group that I know of. They might be able to help you out:
http://www.chess.com/chessgroups?keyword=geezers
I am looking for a tournament to join, in my level. How do I find one? Thank you! Dolly