You can still use Activity.AddMenuItem.
If the device doesn't have a menu button (most device do not have one) then a three dots button will show in the top right corner of the action bar. Otherwise the menu will be opened by clicking on the menu button. I guess that your device is considered a device with a menu button so you don't see the three dots button.
Note that StdActionBar and TabStrpViewPager are not related to the menu.