Hello. I am only working with Express Checkout but there is a need to provide 3rd pary API permissions for external cart vendors. I am currently setting this up for our new mobile web.
Once you have a business account I don't think there are any additional steps for the account to be active. But you do have to allow/provide 3rd party access. It is my guess this is what your cart provider is asking for.
The core of 3rd party API access can be found in your business account's profile. Select the 'My Selling Tools', then in the 'Selling Online' section there is an API Access topic. While this is the main setting it is possible your particular cart provider might need additional setting changes. Hopefully they have a document explaining what their specific needs are.
I hope this suggestion helps. Best of luck.
