Disable items in an NSPopUpButton

I have a NSPopUpButton in a non-document based app where you can select 15 items(video mpaa  ratings). I would like to have  a couple of "headings" that are disabled.  I believe what I need is:

- (BOOL)validateUserInterfaceItem:(NSMenuItem *)item
{
    NSLog(@"Validating Menus");
    //int tag = [item tag];
    if ([item tag]==1 || [item tag]==2)
    {
        return NO;
    }
    return YES;
}

but it is never getting called. I think I am missing a basic piece of info to make this work. I have read a bunch of info and cannot figure out how to get this called in myAppcontroller class. Any help would be appreciated.

Thanks,
 Todd

UPDATE:It was a simple mistake. My pop up button had no action set. I was just grabbing the title from it. I created an empty action and connected it and the - (BOOL)validateUserInterfaceItem: is now getting called.