We add the following registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\AssemblyFolders\Altiris SDK. The AssemblyFolders key is read by Visual Studio when presenting the user with possible references. In our key, we provide the path to which SDK .NET assemblies are installed to. Typically this is C:\Program Files\Altiris\Altiris SDK\Assemblies\. So, any assemblies that are in this folder or nested subfolder should be loaded by Visual Studio.
I have installed Credential Manager Tp4 and I do see that for some reason Visual Studio does not list these assemblies as it should. It seems like a Visual Studio issue, but we will need to investigate.
I am afraid the only alternative right now is to browse for the assembly in \Altiris\Altiris SDK\Assemblies, which is not ideal when sharing the solution or for build machines which may not have the same path.