Thanks for your post and apologies to hear that you've not received your refer a friend credit
There is a specific process in place for referring customers. The existing account holder needs to pop in their friend's details via this page here: Refer A Friend
This then generates an email to both the existing and new customer confirming the referral. The Friend then needs to join via that email. Unless this is followed, no link is made between the 2 accounts.
Have you still got the email showing the referral? If not check with the person who referred you. If they have this we can look into this further for you.
I have sent you a private message with some information on where to go from here. Just click on the little plum envelope at the top right hand side of the page to access your inbox.