If your credit card payment option is greyed out, you can go to Menu > Settings in your POS and enable it under the "Credit Card System" section.

If you had it enabled before but it is currently greyed out, it's because you either cleared the cookies or are using a different computer or browser than normal. The POS settings are saved in your browser cookies, so they do have to be set up if any of those occur. This also applies to assigning cash drawers, and is the reason why you are able to have different cash drawers saved on different POS stations in your store.