I figured out that this is just a bug in XBMC. It is mentioned here, and in serveral other places:
http://forum.xbmc.org/showthread.php?tid=157211It is only a problem if you install VoxTools using the Repo, and then uninstall it, and then try to install it again. Because XBMC "forgets" what is in the Repo, but "remembers" that it already scanned it, it thinks the Repo is empty but doesn't rescan it, or worse, it sometimes shows other, random Addons in the Repo! I'm not sure how that happens.
Anyway the solution for a user is simple. Do a "force refresh" from the context menu for the Repo.
If I get around to it, I'll also create a "new" version of the repo, and update the md5 tag, and that will probably fool XBMC into refreshing the info.
I was worried that this was going to confuse new users when they tried to install VoxTools as shown in the youtube tutorial, but it should not actually be a problem if you are installing the Repo for the first time.
I guess I can stop panicking now.