1) You don't have a <<payment gateway>> activated
Solution: Go to Membership >> Payment Gateways and activate at least one gateway. (Either check the box and apply the toggle activation action, or simply mouse over and click Activate.)
2) You don't have an <<access level>> set for a subscription plan.
Solution: Go to Membership >> Subscription Plans, click Edit for one of the plans, and drag an access level from the right hand side into the "Drop here" box under Membership Plan.
3) You don't have a <<price>> entered into the subscription plan. Please note that free plans require a 0.00 be entered in for the price.
Solution: Go to Membership >> Subscription Plans and make sure each plan has a price in it.