if you bought it direct from Cakewalk, you should be OK since you already have an original Cakewalk account (as it seems those accounts cannot be created anymore). Just sign into Command Center and your stuff should still be there.
If you only got a registration code from a 3rd party seller, I would hope that it can still be redeemed, but I haven't seen that mentioned recently. The Cakewalk store has been closed since last year.
If you run into any technical issues with the website, you should be able to reach support at "support@cakewalk.com".
The new owners haven't said anything about re-releasing any Cakewalk instruments yet, so that bit is still up in the air. So far they have only released the Cakewalk by Bandlab DAW (for free, no store yet).
Sort of wished I had grabbed Rap Pro before it was too late.