Here is their description of the referral system taken from Receive Links FAQ page:
The subscription bonus is equivalent to having a paid account for free. This in turn also bumps up your referral bonus level.For each account (which includes all the sites in this account) we will give you 5% to 15% of the Points of the new account you have referred (this is not deducted off them). This lasts for the life time of your and the referred account. If the Points of the referred account goes up (or down) so will your referral bonus. Also we offer a subscription Bonus for each set of 10 sites that have over 500 points.
How do you know when I have referred someone? Every account has its own unique referral link which can be found in the referral details box which is located on your 'Manage Account' page. You can advertise Receive Links using this link and if someone clicks on it we give them a cookie so if they sign up anytime within 30 days after you will be counted as their referrer providing they do not delete the cookie or come to the site on a different referrer link.
So how do you do this? First sign up at receive links
Then look for your referral ID under manage account. It will say Your Referral Link is ... [your referral url].
Then promote that url any way you see fit. forum sig's, blog posts, dedicate webpages such as reviews,etc. and whatever else you think of.
Once others sign up and gain points you automatically get points assigned to your account. You can now use those points to get links to any URL you want as long as it's cached in either G or Y!.
Good luck in your link building!