Hey Badzav...
I used to use Sonar 6 Producer too! I found this link...
http://www.cakewalk.com/Support/product.aspx/SONAR-6 It isn't much help because most of the links are dead.
The usual support page is here, with phone numbers,
http://www.cakewalk.com/Support/contact/default.aspx#ts-policies But Sonar 6 is no longer supported. I'm not sure what they'll tell you. Still, it seems like being able to install the products is something they would ensure you could do.
Did you try logging in to your Cakewalk account and checking the registration codes on that page? I just logged into mine and my codes don't look like yours anymore. For example, they have no dashes in them. The registration code is 4 sets of alpha-numeric separated by dashes. Maybe your problem is that they have updated their system for serial numbers and registration codes and you are still trying to use the ones from way back(?).
If you go here...
http://www.store.cakewalk.com/b2cus/ and click LOGIN you should be able to log in to your account and view the registration codes and serial numbers for all of your products. If you can't log in to your account, perhaps support will support that... :-)
Also, Support may be closed as it's New Year's Eve.