Whats the trick to castling on chess.com  I just moved my rook over two squares but it didn't do it.  What's the trick?

have to move the king not the rook.

Castling is a king move. 

"This is a move of the king and either rook of the same colour on the same rank, counting as a single move of the king and executed as follows: the king is transferred from its original square two squares towards the rook, then that rook is transferred to the square the king has just crossed."

If castling is legal, move the king and the rook will automatically complete castling.

thanks I thought I had tried that.  I will try again the next time.