Payment Gateways in Membership Plugin

Hello, I am trying to create a membership site with 3 levels. Stranger, Free, and Paid. Free Members are only allowed to access a listing page and Paid Members can access everything. Paid members pay $19/month. I activated two payment gateways (Free Subscriptions and Authorize.net AIM). After I filled up the registration form in Step 1, I only get the subscribe button for Free Subscriptions. There is no subscribe button for Authorize.net AIM. I wanted to have the Authorize.net payment gateway for paid members. I attached a screenshot for reference.

Am I doing this correctly? Or is there another way of doing this?

Thanks,