db2 express c mac osx
You do not want to change DBINSTANCE. All that does is change what instance is the "main" one for your OS. And you only have one instance from what I understand (named db2 I believe). I'm betting this has more to do with permissions as you are attempting to run db2sampl, which I believe creates the sample database if I remember correctly. Instance owner should be able to run this without any issues.

If I were you, I would run db2sampl as instance owner and then perform your necessary GRANTs for db2-user once the database is there.