I haven't used a trial version on Nero in so long I'm not sure that one is needed...is this the first time you've tried the trial version?
It does ask! I also did not expect. I had version 5 before, uninstalled and tried with this one. Installation of the trial version prompted me to uninstall previous version which I accepted, rebooted PC and - now the code!!! I'm puzzled.
Did you use the Nero cleantools from their website to clear all remnants of other versions from your PC?