Import from WP-eCommerce to MarketPress Question

I've watched the videos on how to import from WP-eCommerce, but it doesn't say how to actually DO it. It points out where to go, but it doesn't say, for example, what do you do in my situation where the WP-eCommerce info is on another site. How do I get that data from THAT site into my new site? It's essentially a whole-site migration, but you do need to provide instructions for me on how to do this.