It sounds like Excel isn't recognizing the addin correctly. The usual reason for that is that a dll dependency is missing. I expect that in this case it's the cygwin dll it can't find.
Can you check that both the python dll and the cygwin dll are in your system path? You could also try copying them to the same place you have the excel binary installed to see if that picks them up.
Depending on your build of python and the version of excel you're using you may also have a dependency on a version of the Microsoft c runtime that's different to the one excel uses. That's not normally a problem, but you need to ensure that can be found as well.
Dependency walker is a good tool for checking these sorts of problems. You may have it as part of a visual studio install, or you can download it for free from the web.
I hope this helps. Please let me know if you are unable to resolve this.