Android Eclipse project can't find basic classes

Hello.

I'm trying to export my Unity Android project to Eclipse so I can add a plugin. However, I can't seem to get it to work. I build with the "Create Eclipse project layout" option, but when I open the resulting project in Eclipse, it's full of errors. Specifically, it doesn't seem to be able to find any of the basic classes such as Activity, Bundle, etc.

I think it worked before but not anymore, even if I make completely new, clean exports (from Unity). It looks like it's missing a library or something. Am I supposed to add a library by hand?

Thanks in advance for any help.