You can add your subscription fee or tokens from the Payments link in the left sidebar. There, you can click the Redeem Coupons tab and enter your provided access code. Please note that this field is case-sensitive, so you'll need to make sure it's being entered exactly as you were given it. The best way to do this is copy-and-paste.
You should see your token balance change in the upper right hand corner of your screen immediately after you click Submit Code. If the code includes your subscription fee, you'll also see a Fee Paid message appear. If the token balance isn't what you were expecting, go to the History tab to see an accounting of all of the tokens you've added and used.
Note that coupon and access codes are only valid for one use.