Android: class not found exception: android.support.v4.app.FragmentPager
You have to add compatibility library by right clicking your project and selecting Android Tools -> Add Compatibility Library. Once its added, clean your project and build again.
i'm using Android Studio Beta 0.8.6 and i had to do it this way
File -> Project Structure
Go to <Your android apk folder>\sdk\extras\android\support\v4 folder and add android-support-v4.jar to app library (or whole sdk library).