You cannot go any further unless you are signed in. You can sign in with a Facebook account to your Groupon login. If you sign in with facebook, you may still need to create a groupon account. However all the information is automatically populated for you. Once ready, hit the SIGN IN button.
Before you enter your payment information, you will see a small box to check in the top right corner. Check this box and you will be able to enter your coupon or giftcard.