To castle, drag your king where it would move if castled, and it should switch positions with your rook if legal.
castling

Do you mean how to castle by moving both your Rook and King? If so, make the move by dragging the King two squares to either the right or left, depending which way you want to castle, and release it over the square the King will end up on after castling. The software recognizes what you want to do and automatically also moves the Rook to the proper position.
EDIT: Sorry, Bacon-- your answer posted while I was writing mine.
how do i make a castle move?