I want to use MoPub for putting ads in my Android app.
I have downloaded the required resources and set-up a library project (mopub-android-sdk) in Eclipse. Now I want to add this project as a library to my Android app. But when I try to do that it gives this error:

Are the mopub-android-sdk files in the same parent folder as your project? This seems like the same problem addressed in this question/answer:
In eclipse, unable to reference an android library project in another android project
Place the referencing project in the same physical folder and see if the error persists.