After you purchase a deal you should get an email with a certificate code attached. The redemption instructions will be listed. It is possible that the email was sent to a junk folder if you didn't see an email come through.
You can also log into your account to see your certificate purchase history.
When Signed in at Open Market Deals click on the 'My Account' Link on the top right. Click on Deal Certificates on the left hand side directly under My Account.