Visual Studio .NET provides the “add reference” dialog box, so we can reference them by clicking them. But, BizTalk Server 2004 installation doesn’t add the .NET assemblies to the “add reference” dialog box by default. So, we have to manually add several assemblies whenever we create the BizTalk artifact such as adapter and pipeline. :-(
BTW, the dialog box lists the assemblies listed under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\AssemblyFolders. I made a simple script for this one. I always add these registry keys whenever I setup the development machine for BizTalk.