Here is some info for those who seek and those who are lost:
The stems licence will show up as Engine DJ Stems on your InMusic profile page, under Product Registrations. There is a serial number too, but I did not need it to test stems on my players.
On the SC Live 4 and the SC6000M players, you need to log in with your profile, which, by the way, you can edit here:
https://enginedj.com/account/profile
The SC Live 4 and the SC6000M will tell you in a pop-up window that stems are available now. I simply hit “Got it!” Then the players rebooted for an update …
For stems I exported the sole track which I have put into stems analysis so far through the Sync manager to USB. I loaded the track … On the SC Live 4 push the Cue button right next to the Shift button and you get four pads to control the stems … on the SC6000M it is the Hot Cue button. Apart from using your profile, no settings need to be changed … at least, this is how it worked for me …
My test track was from The Fairy Queen by Henry Purcell, the track consists only of melody, no vocals, no drums … so my first test was not very informative. 
Thank you, Denon DJ team, I will happily try out this cool new feature. 
