I am assuming your menu.xml has an appropriate menu closing tag. Do you have this xml file stored in your res/menu folder? Can you verify that onCreateOptionsMenu is called?
On Android 2.3 and lower you will have to press the menu button, whereas in later distributions it should be displayed in the title bar. Since you are using a TabActivity I assume you are writing an app for Android 2.3.
Perhaps when you press the menu button, the options menu of the specific tab activity that you are in (e.g. LogInActivity) is called, and not that of its parent. Try putting this code in every subactivity:
public boolean onCreateOptionsMenu(Menu menu)