The code responsible for opening the drawer when a card sale (or other type of payment other than cash) is only run when receipt printing is turned on. In order to get the drawer to open therefore you must select print receipt before finalising the transaction - click on the picture of the printer first as shown on this screenshot (bottom left corner) - now when you complete the sale by card, the receipt will print and the drawer will open. At present, I think this is the only way to do this.