Before you can start earning affiliate referral bonuses for linking to books at the Smashwords Store, you'll need to create a Smashwords account (or log back in). Once you're in, head over to Account / Affiliate Management to enable the Smashwords Affiliate Marketing Program on your account.

In the meantime, you can learn more about Smashwords, and review the Smashwords Affiliate Marketing Program Documentation and Smashwords Affiliate Marketing Program Agreement (Terms of Service).