The "Redeem Code" option is only for people who are completely new to FITE. It will be available in the week after you sign up and can be used if you have been referred but have NOT signed up from the link provided and have not received the 10 FITE credits. If you have received the referral credits on signing up there is no need to redeem the code.
You can redeem the code only from the FITE mobile app. Go to your account from the menu on the top of the home page and select the option to "Redeem code" from the new menu.
If you want to know how to use a Coupon please view this article.