General info for using someone's referral code:
Go to the website via the referral link, and proceed as normal to sign up for a plan. There is no spot to enter a referral code-- it's set by starting the process via a referral link.
If you've entered email address info before on the FreeUp site, be aware that the page may default to that previous info, and may not credit you with a referral bonus. I think you can fix this by logging out of the pre-existing account information, and starting over with the referral link.
Extracted from the FreeUp Mobile FAQ:
Someone signing up via a referral link will "receive $5 when they join and (the person supplying the referral will) get $1 plus 5% of what they earn each month when the recipient signs up by using your link"