If the scanner you are trying to add NXDN to already has the DMR key (presuming you added that), you should be able to log into MYUNIDEN, and click on the "All My Scanners" link. If the scanner is in your list, just click on the link for 'Purchase NXDN Key". I have two 436HPs, so I have to make sure to choose the one I update.
As bearcat noted, if the scanner is already registered for you, you don't need to re-enter the serial #, ESN, or checksum.
Take a close look at groups on the keys. I typo'd one of mine the first try when I misread a character.