I am experiencing the exact same problem.   No matter what I have tried, the add-in simply will not appear in VS2008.   I also have the hotfix.   AND, as far as I can tell, using the upgrade utility from outside of the IDE DOES NOTHING.

The add-in basically calls off to the same code library that the standalone Upgrade Utility uses, so if that isn't working, I don't think that the add-in would either.  If course, since you can't see the add-in anyway, this is obviously a moot point.  Unfortunately I don't really have a solution for this, since add-ins are pretty finicky with reproducing or tracking down issues.  You should probably notify Developer Support with this, as well as as much information as you can provide regarding your installations of Visual Studio, NetAdvantage, etc.

If the standalone upgrade utility isn't working, you could do what I do for quick test projects and simply open up the project file in Notepad and do a find/replace on all the version numbers; for example, you would replace all instances of "v7.2" with "v8.3" if you're upgrading from NA 7.2 to 8.3.  You may also have to clear out the licenses.licx files.  I realize this is a pain for solutions that have multiple projects, but it's the best workaround I can think of.  Of course, if you need to upgrade a web project, you'd also have to do the same things for the Register tags on each page.

-Matt