I added instrument credits for additional artists with “relationships”, but didn’t find out how to do so for the main artist. The same problem occurs with writing credits for non-cover tracks.
Can someone help?
I hope my newbie questions are not too annoying, but I’m still not very familiar with editing musicbrainz. I managed to add only one of a bunch of Harri Stojka CDs and struggle with a lot of details 
Thanks in advance!
It sounds like you’ve already found the relationship editor because you’ve added instrument credits for additional artists. If I understand correctly, you would add the instrument credits for the main artist the same way.
As for the writing credits, they are actually attached to works through relationships. From the same relationship editor, select a “work” for each track (or create a new one if there is not already one existing). Then you can attach a writer credit to the work.
Got it!
The guitarist has a relationship to the album artist.
Now lets try a related work … but I think, I have understood.
Thank you!
Glad I could help, and welcome to MusicBrainz!