If you made the Add-In available to all users, then it doesn't show up in
the outlook COM Add-Ins dialog.  You can see it in the registry here
though -
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Outlook\Addins\YOUR_ADDIN

If you make the Add-In only available to the current user (there is an
option for this in the wizard), it will put the registry entry under
"HKEY_CURRENT_USER" instead, and you will be able to see it in the Outlook
COM Add-Ins dialog box.

Also, because C#.Net and VB.Net create "Managed" add-ins, they are called
via a microsoft dll called "mscoree.dll", so it might show up as "mscoree"
in the list, and you will not be able to add it to the list by selecting
your add-in's dll.

The only way that you can get it to behave as expected in that list is to
build an "Unmanaged shim" which in turn calls your add-in.  You can build
this shim in C++ 7.1 (VS2003).  It is explained here -
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnoxpta/html/od
c_comshim.asp

Matt.