Can't I just add the Last.fm data to the N900 version?
Since Vagalume is free software, there's no way to prevent you from doing that, but note that you might be violating the Last.fm terms of service. The Vagalume team takes no responsibility for that.
If you really want to do that, please don't use the Last.fm API key that comes with Vagalume. That API key is necessary to identify the client and you might be harming legitimate Last.fm users if you violate the terms of service using the Vagalume API key. Get a free key here.
What?! I don't understand any of what you said. All I want to do is stream some music. I have a Libre.fm and last.fm account. Why won't it accept my login and just play? And if I need a file to play it, why doesn't the app retieve it for me? I don't even see any place to download this file. Since the file is needed, why not give instructions on how to get it, then import it into the N900? Or put "for rocket scientists only" in the details...