JamminR,
Actually, "ulx adduser" can be run by anyone with superadmin access, however, only the listen server host, or dedicated server console is allowed to modify access of users who are already superadmins (for security reasons, to prevent a rogue superadmin from stealing all access for himself.)
Danny,
Make sure you are a superadmin, listen server host, or at the dedicated server console, and type "ulx adduser" in the console for more help.