First, find the "Earn" page, highlighted below, and click on it:
Select the Referrals page, as shown below:
Enter your referral code in the box at the top, like this:
Click the submit button and your referral code will be entered and activated, and will then look like this:
Remember, you can only use one promo or referral code on your account, so make sure you've got the right code or your referrer will not get their earning bonus.