Noztra,before being able to use in-game commands, you must add yourself as superadmin (if you're host) using the server console and the adduser command, or, to have all addons see you using normal Gmod methods, add yourself to your gmod/settings/users.txt (that file has little to do with ULX, other than we read it at server startup)
Our ulx readme.txt in /addons/ulx and our FAQ for how to add oneself as admin point to a GMOD Wiki article that USED to exist on how to edit the settings/users.txt file.
Unfortunately, Garry decided to wipe the old wiki.
None of us here have time (or patience) for recreating the article there.