Go to https://nook.tapfiliate.com/login, then click the "Forgot Password" link.
Enter the e-mail address that you used to register for a Nook Affiliate account.
Check your e-mail that was sent by the "Nook Affiliate Team", click the password reset link in the e-mail.
Enter a "New Password" and confirm the new password you created in the next "New Password" box. Click the "Change Password" button.