I've downloaded and installed Photoshop CS5.1, entered my serial no. (came up with large tick), then enter adobe id and the next page says
"We are unable to start your subscription of Photoshop cs5.1 extended subscription edition"
Welcome to the forum.
This might be a case of needing to call Adobe Customer Support.
The "subscription" version is quite new, and probably most here have never used it. Also, with S/N issues, the best that the majority here can offer is based on reading of problems that others have posted of.
Hope that someone here has a fix for your issue, but I'd be planning on making that call to C/S (not Technical Support), and having all documentations, invoices, etc. handy.
Are you installing the
and entering a code for the normal Photoshop product by any chance?