Eu criei a classe 
public class MainActivity extends Activity {
}
e inclui as tags  no Manifests 
  <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MENU"/>
                <category android:name="android.intent.action.LAUNCHER"/>
            </intent-filter>
        </activity>mas ao executar ocorre o erro
Error running 'app': Default Activity not found
 
            