I'm not an advanced user in Outlook, but maybe the menu only shows emails you have already used in the past?
Like in IE or Firefox, if you type something in a textbox it sometimes saves so when you come back you press the letter it begins with and its there.
I know that it's not IE or FF, but you know what I mean! Maybe it's like that? Try sending some emails first then try seeing if they are in the dropdown menu? Or maybe it just doesn't have it.
Sorry for naff answer.