Nobody here in the forum can help you with such an old application. CS5 is being 15 years old and is unsupported on new operating systems. Adobe has also stopped the supporting such old versions. The activation servers for CS5 are also has been shut down. This means that it can't be activated anymore.
Actually, the CS5 activation servers are still alive - but probably not for much longer. CS4 and back have been cut off.
But yes, this is 15 years old and barely hanging on to life. If the OP really means register the software, not install or activate, don't bother. There is absolutely no reason to register it now, you won't be getting anything for it.