To the best of my knowledge the program is not longer supported by Microsoft. You can however, save the files in 2003 format and use the TW that way. It works but is not what I consider the easiest way to work with it.
I am in a state of shock that MS would not continue using this most wonderful add-in and one that is, as far as I am concerned, one of the better creations that ever came out of Microsoft. Shame on those guys for dropping an add-in that really showed how powerful Excel can be with such a great add-in.