Linking assembly problem

Hi all,
I can not found the Nortwoods.Go.dll assembly reference when I open the ‘Add reference form’ in VisualStudio (.NET tab) even if the assembly is registered in GAC.
Why?
In my team we use SourceSafe and there is a relative path problem if we use the reference browsing it from the file system.

Could someone help me please?
Thanks in advance.

Maurizio