Referral Codes can be used when signing up for a new account. You can only use one referral code for your account, and only during the initial sign-up process. We do this to make sure that referral codes are being redeemed when they are supposed to - upon the creation of a new account.
After completing signup, you will notice the 'Promo Code' field in your profile settings. However, you cannot enter Referral Codes in that area as they are different from Promo Codes. Entering your referral code in the promo code area will not work after signup. Again, you can only redeem a Referral Code when signing up for a new account.
If you have both a Referral Code and Promo Code, you can insert your Promo Code in your Profile Setting after completing the sign-up process.