assuming you know the password to your RCON for your server.. if not.. you really should... contact your server administrator (people you bought the server from) and ask them
do the following steps in YOUR GAME CONSOLE
rcon_password <password> *where <password> = your RCON password for your server
rcon ulx adduser lolololz superadmin
That person is now added... and you should get a message from ULX confirming that.
Editing the default user.txt file is a way too.. but personally I think the RCon way is easier.