Hey @soundsgood,
Unfortunately Keyboard Maestro doesn't have a mechanism for directly dealing with pop-up menus – the user has to resort to AppleScript UI-Scripting to manage them.
In this example the way to manage is to create a shortcut in the System Prefs > Keyboard > Shortcuts > App Shortcuts > Mail.app > <Exact format of sub-menu item in pop-up>
I have 7 keyboard shortcut for directly selecting different addresses:
I also have a macro to pop-up the menu for type-select to pick up a few I don't bother to have keyboard shortcuts for:
Pop-up Menu ⇢ Open ⇢ Outgoing Message ⇢ FROM.kmmacros (6.2 KB)
-Chris