If you have been given a coupon code, this can be entered on the subscription page for any completed profile.
To use a coupon code when subscribing a new profile, this can be entered in the final stage of the profile creation process.
To use a coupon code on a profile which is already subscribed, you can enter this by heading to 'MyBilling' from your dashboard, then 'MySubscriptions' and then navigate to the profile you would like to use the coupon on and click 'Edit'.