Sorry, I wasn't clear, I meant that you need to run Step 3 on the Mirror before
you activate the mirroring session.
Actually, just thinking about this some more, when you manually failover, is the user present in the database, but as an orphaned user? If so, you can just run
sp_change_users_login @Action='update_one', @UserNamePattern='your_user', @LoginName='your_login'