Ah, I think I see what’s happening then; it’s because basically the importers are currently written assuming only a single user. I need to make some pretty significant changes to support simultaneous users on the same blog unfortunately.

Off the top of my head, the only way I can think that you might be able to make this work is to import everything on a different blog entirely, then export all of your posts/media, and import that WXR/WordPress file into the main one.

I’d have to have a think about exactly how to support multi-users.