Why this error message ?
This is a Lint : Instantiatable error message which says that "Classes registered in the manifest file are to be instantiatable" i.e. All the Activities, Services and Broadcast Receivers that are registered in AndroidManifest.xml must be public. It must have an empty public constructor and if it's and inner class, it must be a static inner class.
Right Click appcompat_v7 project folder -> Properties -> Lint Error Checking, and look for Instantiatable and change the Severity from Fatal to Warning!
Clean and Build your Project folders and you would see that this error message is not just a warning.