The internet is a great place, there I was scratching my head trying to work out how to get my .NET assembly to appear in the references section of an Integration Services Script Task when I stumbled across Daniel Read's helpful Fun With SSIS, Script Tasks, and the GAC (Global Assembly Cache). In a nutshell, and I'm sure Daniel puts it much better than I can, to get an assembly to appear in the Add references dialog you have to both register your dll into the GAC AND copy the file into the appropriate framework directory - yeuch!

From the Professional SQL Server 2005 Integration Services book SSIS can only use assemblies located in the .NET framework installation folder for version 2.0

So far this is the only yeuch moment I've had with SSIS