I think you should check two things first
- Is the Google Play Service project or library added to your project ? If not, check this link http://developer.android.com/google/play-services/setup.html
If you compile for android version < 11, you need to use supportFragmentManager instead of the normal FragmentManager :
mMap = ((SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map)).getMap();
In the last case, you also need to update the class used in your XML to class="com.google.android.gms.maps.SupportMapFragment"