Often developers think that if they create a strongly-named assembly and drop it into the Global Assembly Cache (GAC) that it should show up in the .NET Add Reference dialog list automatically. Let me tell you that this is simply not the case. Even Microsoft did not pull off this magic. This is due to the directory structure of the GAC that puts each assembly into its own unique directory using parts of the strong name. That is how they enable side-by-side versioning in the GAC (but this may change in future versions).
In any event, the point here is that you will have to have two copies of your shared assemblies (i.e., assemblies in the GAC) on your development box, one in the GAC and one in a separate location for referencing at compile time. Microsoft does this and so should you if you want to make your life easier. See
C:\WINDOWS\Microsoft.NET\Framework\v1.0.3705 for their compile-time assemblies (notice the XML documentation files included there as well to enhance Intellisense). These are the ones you see in the .NET Add Reference dialog, but the copies the applications that you build use at runtime are actually located elsewhere in the GAC directory structure.
I say all that to point out that the steps you took above will make displaying your shared assemblies in the .NET Add Reference dialog quite simple--just drop them (and their XML documentation files if available) into the Third-Party Tools folder after you put them in the GAC and they will neatly appear in the dialog. That's just one extra, easy step to make your life easier. And if you're distributing shared assemblies, you should make your customers' lives easier by automatically putting your assembly into a folder that is under the AssemblyFolders key during your assembly's installation (as you can see aspNetEmail has done in my screen shots).
One last note is that you may need to restart VS.NET after adding new assemblies to the Third-Party Tools location before they will show up in the .NET Add Reference dialog listing.