You need to have Manager permissions in order to activate a swipe card for an employee. To do this, browse to the "Manage" section and then click on the "Swipe Cards" section in the left sidebar.
Click the "Activate Card" button, and the POS will prompt you to enter the employee's timeclock ID.
Once you enter the ID, it will prompt you to swipe a card through the card reader. This will assign the card to the employee, and you'll see a green confirmation message flash at the bottom of the screen.