Android support v7 can't find ActionBarActivity
Delete the support v4 jar file in your project/lib folder.
You can have it there and work with the support v7 lib, but v7 already has a v4 lib and they must have the same version to work.
Remove the v4 jar file, then add the v7.
You have to be in the new version, Android.Support.V7Inherit from this class Android.Support.V7.App.AppCompatActivity
public class MainActivity : Android.Support.V7.App.AppCompatActivity
New v7 appcompat library:Added ActionBar to allow implementation of the action bar user interface design pattern back to Android 2.1 (API level 7) and higher. Use of this class requires that you implement your activity by extending the new ActionBarActivity class.