That's odd. Anyway if you have the update installed do this:
The default way:
1) Run your 8.1 msstyle in WSB Beta in Ribbon mode
2) Click Add Class
3) Type in ImmersiveStart::Menu
4) Save the msstyle
5) Reopen it and you should have a new class Unknown > ImmersiveStart::Menu
6) Add a fillcolor to ImmersiveStart::Menu > PopUpBackground
7) Copy/Paste or just enter all the properties and values from Unknown > ImmersiveStart::Menu > PopUpItem image in the 8.1.1 aero msstyle (except FILENAME:IMAGEFILE) to the one in your msstyle.
Add the image from Unknown > ImmersiveStart::Menu > PopUpItem in the 8.1.1 aero msstyle to the one in your msstyle by exporting it to your desktop and then adding it using the Add Property button (filename, imagefile, choose the image). The image has to have a solid color
8) Save it and you'll have the Start Screen PopUp Menu
A Better Way ( found by Albertt To)
http://vistastylebuilder.com/forum/index.php?topic=2069.msg11858#msg11858