I was wondering the same thing.
It looks like you need to get "Windows Live Photo Gallery", from Windows Live Essentials.
I've just installed it and it seems to be locating all of my custom tags.
It seems that you can create tags and add them to the existing list (for future use).
Now all I have to do is see how easy it is, to add them to the 3000+ pictures, that I actually want to tag.