When you sign up via the buyer referral link, you'll get a $10 credit. However, this can only be used on card purchases $11 and above. This ensures the promo code isn't misused. If you try buying a card worth $10 or less, your order will not go through.
The $10 Credit does not work with Slab Packs, it's only for cards in the Marketplace.
You will be able to find the promo credit for the $10 credit at arenaclub.com/rewards. Please note the promo credit will expire 7 days after creating your Arena Club account.