In Point of Sale, tap "Other Tasks" and then "Find or Register Patron".

Search for the Patron by typing their name.  If the Patron is not already registered, then you should add them before proceeding.

On the "Advanced" tab, click the "Swipe Card" button, and then swipe the membership/loyalty card you wish to register to the patron.

NOTE: The "Swipe Card" button will only be enabled if there is a Magnetic Card Reader attached to the computer. If no device is available, then you can manually enter the card number (if the card has it printed on to the back) by tapping "Edit" and then typing in the card number from the card.