It's still there. Just choose custom from the drop down time menu and a "rating options" button pops up. You might not want to use it though after reading this:
http://www.chess.com/forum/view/livechess/new-logic-for-auto-pairing-and-seek-graph
And this:
https://support.chess.com/customer/portal/articles/1622811-how-are-seeks-matched-in-live-chess-
What happened to the feature where I can set a range of opponents ratings to play?