Where do I set my PayPal API details in Membership plugin?

I've got quite far with the Membership plugin but have hit a brick wall. When I go ahead and test payments to the two subscriptions I've set up on my test site here http://dev1.digitaldavidson.co.uk/lesson-one/ , the payments go through ok (Sandbox account), but the users status doesn't change - i.e. they don't become a member, and therefore don't have access to the levels they should have access to.

I can obviously change this manually from the dashboard but surely this should be automatic??

I expect there's an issue with the API connection between the test site and PayPal - i.e. PayPal isn't telling your plugin that a payment has been made.

I can't actually see where to add in the PayPal API details - do I need to put these somewhere?

Hope you can help - I like the look of the plugin and think it'll do everything I need, once I can get this bit to work.