If you have a MyUnyte account, go to the drop-down menu under your name in the upper right corner of your dashboard. Select "view billing" and then add your payment method.
Use this link to access the "My Saved Cards" section of your iLs store account. If you're not already logged in, this link will take you to your login page. Remember, your iLs store login is your email address and whatever password you set up. This password can be changed using the "Forgot Your Password?" link.