After much searching I've discovered the issue. It has to do with the fact that .NET is managed code and therefore cannot create a self-registering COM object. The way around it is a very complicated task of creating a unmanaged code (called a shim) that does the registering of the managed .net com object. There are some very long and detailed instructions at MSDN ( http://msdn.microsoft.com/library/de...dc_comshim.asp ). I'll give it a try and if successful I'll post the results here. Although it doesn't appear that to many people are interested in creating com add-ins for MP.