How to use Marketpress on Multisite?

I have multisite installed and have multiple subdomains set up. Each subdomain owner will be selling downloadable music files, but we need all music files, across every subdomain and on the main domain, to be in one global shopping cart. One of our developers that set up this site for us has tried to implement this, but has only managed to get a separate cart on each subdomain, rather than all products in one cart. From what I understand in reading about MarketPress, what I need to do is possible but I need to confirm that and figure out how to do it. At least something I can pass on to the developer so he can try and get it working.

Thanks!