There are multiple ways.
Highlight a midi track, open the inspector, and you should see your midi routings, then midi channel option, and below that will be a blank box or perhaps it will say “no drum map” or something like that.
Click on that box which will give you a list of all your drum maps. If you don’t have any, you still get the option to open “drum map setup.”
It can be quicker to open this from the drum editor depending on your workflow. Alternate is to open the drum editor page, and in lower left you can click on “drum map” or “general midi” and you can open the drum map set up from there.
Anyway once the drum map setup page is open, you can create your drum maps. Once you have created your drum map, in the upper left part of the drum map set up, click on Functions> dropdown> save. Once you do this your drum map is always there no matter what project you open.
Poof and its gone…yes you learn this by trial and error.
I’m afraid the Cubase Drum editor hasn’t received many functional updates in years. There have been numerous requests for simple windows management things like move multiple tracks at once or even simply delete the tracks you don’t use. At the moment the only thing you can do is move one track at a time, and simply blank out the name of a track instead of making the track vanish. You do end up wasting a lot of time moving, deleting names, etc.
On the bright side, many other DAW’s don’t even have a drum editor. It’s all relative.
By the way if you use lots of Toontrack mappings, this can be helpful.