I created a new admin user in order to get a different standard username for the admin than "admin". The plugin panel isn't visible for this new user though. He definitely has the role administrator. When I login as the intial admin I can see the panel. It's almost a clean installation. I just activated the blog-network (multiple-blogs within one physical installation). Unfortunately I don't have access to a database backend like phpmyadmin.
EDIT: I just saw that the inital admin is Super-Administrator, is there a way to make a new user Super-Administrator or change the username of the Super-Administrator?
EDIT: Okay, problem solved. Found the checkbox for making an admin Super-Administrator!