Looks like you found a problem we were not aware of.
Our printers somehow printed a batch of incorrect serial keys for our US Retail market. At least I hope it's only the US Retail market.
For anyone else who finds they are unable to install NeroMix, and when they launch the Nero StartSmart and under the 'Nero Product Center' the serial key reports as Nero Enterprise Edition, these are the steps you need to take to resolve your issue.
1) Scan in the receipt (Proof of purchase)
2) Provide the current 'Nero Enterprise Edition' Serial key you are using
3) Send an EMail to Techsupport@nero.com
and request that you be provided with a replacement Nero 6 Ultra Edition Serial key.
Note: Once you are provided with a new Serial key, your 'Nero Enterprise Edition' serial key will be "BlackListed". Which means upon the next posted upgrade on the Nero Web Site, that serial key will no longer be accepted.
Sorry for the trouble this may have caused anyone.