Hotkey to menubar app in Mavericks
The problem in this case seems to be that Mountain.app is not a "proper" part of the menu bar, at least as far as "System Events" is concerned. 

You can see this also when you use (fn+) ctrl+F8; this should put keyboard focus on the status menus, so that you can navigate them with the arrow keys. 
Some extra menu items (e.g. iStat menus) can be selected this way, while others can not. This, I believe, translates to accessibility with AppleScript via "System Events".