Hmmmm... that's odd. I was under the impression that OSP allowed a user to install it on as many PCs as he wanted. That's Ubisoft's official take, at least. So I'm really surprised that it's not allowing the key.
Unfortunately, any and all tech advice could be viewed as circumventing copyright protection, so I won't give any. Instead, I'd refer you to Ubisoft.
First, I'd refer you to
http://www.ubi.com/us/support/ for support. If you click on the FAQ links, there's a tab where you can access their "Ask a Question" service... you'll need to log in.
Second, their tech support number appears to be (919)460-9778, open Monday through Friday 9AM - 9PM. It (and other phone numbers) can be found at
http://www.ubi.com/US/Support/InternationalSupport.aspx .
Good luck.