Having problem with R.menu

I've been trying to learn about menus in android and just got this example.

Code (Text):

1. package de.vogella.android.menuitem;
2.  
3. import android.app.Activity;
4. import android.os.Bundle;
5. import android.view.Menu;
6. import android.view.MenuInflater;
7.  
8.  
9.  
10.  
11.  
12. public class MainActivity extends Activity {
13.     /** Called when the activity is first created. */
14.     @Override
15.     public void onCreate(Bundle savedInstanceState) {
16.         super.onCreate(savedInstanceState);
17.         setContentView(R.layout.main);
18.     }
19.    
20.     @Override
21.     public boolean onCreateOptionsMenu(Menu menu) {
22.         MenuInflater inflater = getMenuInflater();
23.         inflater.inflate(R.menu.game_menu, menu);
24.         return true;
25.     }
26. }

but somehow for some unknown reason to me the R.menu gives me an error on the word menu and it say it can't be resolved, any idea ? thnx in advance