Setting up a free membership plan is can be completed in 'edit member plans' and using the subscription form generator.
1. Under 'Master Settings' choose 'Edit Member Plans'
2. Add a new plan that is for $0.00 and is for Non-Recurring Billing
3. Note the number of this new member plan
4. Create a subscription form using the 'Subscription Form Generator'
5. In the Subscription Form Generator - The 'Form Type' can be set as 'Medium' or 'Short'.
6. In the Subscription Form Generator - Check the box for the free member plan.
7. Paste the CF tag for the subscription form generator on a new content page in the 'text or html' box and add the page.
8 Under 'Master Settings' choose 'Edit Custom Settings'
9. Add the content page number or keyword rich URL where the subscription form was added to the 'Complimentary Payment Pages' field
Questions about the complimentary payment pages in Custom Settings as explained here
More information on setting up member plans can be found in this tutorial: How to Set Up Member Plans