Blue, thats incorrect unless you're running server from a game client (non-dedicated server)
You're automatically superadmin if running server from game.
Frank, define "doesn't work". What are you trying to do that doesn't work?
What errors? Using ULX 3.51 I presume?
[Edit] - Additionally to answer your question about file..no, we don't recommend manual file editing of ULib's users.txt
However, ULX imports from the normal Gmod file that it uses in gmod/settings/users.txt
See our FAQ for links of how to add yourself as admin. It points to the Gmod wiki for the settings/users.txt