You never will be able to activate it. Adobe has taken the activation servers offline for that version and the one following.
You might want to take a look at this, which covers your situation:
Hm, I notice on looking deeper that the wording there has been changed. It definitely used to say "or older". Perhaps it no longer covers your situation. If that's the case, I'm sorry but there simply may be no way to run Photoshop CS any longer.