It is giving you error because you have defined it outside the method. Your code
IntentFilter filter = new IntentFilter(BluetoothDevice.ACTION_FOUND);
registerReceiver(mReceiver, filter); // Don't forget to unregister during onDestroy
is not inside any method, Just create a onResume() method like below and move your code in it.
@Override
public void onResume()
{
super.onResume();
// Register the BroadcastReceiver
IntentFilter filter = new IntentFilter(BluetoothDevice.ACTION_FOUND);
registerReceiver(mReceiver, filter); // Don't forget to unregister during onDestroy
}