Move the king two spaces in the direction you want to castle. In 960, click the king and then the rook.
Edit: Also, maybe there is a reason you can't castle? You can't castle in check or through check. You can't castle if you've moved either the king or the rook you want to use to castle.
How could i do this movement?? I know the rules but i can not do it. Should i move the king? Tower? both? Thanks