I bought a Maschine Plus but I don't understand how to license it with my Native Instrument profile
Take a look at this page:
Once you've found your serial number, register it in Native Access by clicking on "Add Serial" in the lower bottom-left.
Could it be you need to download Native Access?
https://www.native-instruments.com/en/specials/native-access-2/
There are surely some papers which came with your Maschine Plus. Read those.