I'm trying to buy an app (NavDroyd) for my HTC Desire but when I hit "Buy" it comes up with two buttons, "Credit Card" and "Cancel". "Credit card" is greyed out so all I can do is hit "Cancel" and go back round in the circle. This is the first time I've tried to buy an app. Can anyone tell me what to do next ? I've tried to help myself but a good hour going round the Android and Google troubleshooters and whatever else they are called has been unsuccessful.