You may notice that sometimes when you install a new addin it doesn't show up in your list of available addins. I had this problem with the SoliNav addin that showed up on a number blogs yesterday. After doing a bit of registry hacking I discovered the problem; SoliNav registers itself for VS.Net 2002, and VS.Net 2003. Unfortunately I have the VSIP installed and I use this version of VS.Net 2003. So here's the solution...
Open up your registry and go to this key (for a single user setup) : HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\7.1\AddIns. You should see a folder (node) here that has some reference to the installed addin. Export this node and edit the resulting reg file. Change the "7.1" to "7.1Exp."
There's also a similar key for addins available to everyone at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio with 7.1 and 7.1Exp nodes