Are there any VSTO 2010 experts here?

I am hoping to use a VSTO Excel 2007 AddIn as a kind of class library
that I can reference from existing VBA projects. However, having built
and published and installed such an AddIn containing a test class, I
can't see how to reference it and get at the class therein.

I also tried creating a class library but Excel returned the message
"Can't add a reference to the specified file." when I attempted to
reference the dll.

Any advice, even if it's just "go and read this" would be greatly
appreciated.