Ok, it fails because your AAC tracks are in Other Media, they need to by in the main music library.
I assume you are using the 3ivx codec, in which case that shouldn't be the problem. And you are able to play your AAC files from within WMP right?
How did you point WMP10 at your iTunes library? Using the Monitor folders option? Or the Add To Library option. Either of these methods should end up with all your AAC music files being listed in the WMP10 music section. They will not be tagged however and only the title (filename) will appear.
Are your files unprotected? Or are they iTunes purchased. Protected files won't work.
From reading the WMP documentation it seems to me that files get dumped in the "Other" category when WMP does not know what type of media they are. There must be a setting that tells it AAC (.mp4) files are music. I don't know of anyone who actually has had to set that up myself, WMP just knew for me.
Did you install 3ivx before or after importing your library...