You need to be on your server at the time you run the command to add yourself as a user through ULX, in which case you shouldn't need the rcon_address set. Regardless, nothing will show up in the RCON box on your Web Control Panel, because rcon is based on individual connections. If you are on the server when you run the command, and the name you enter matches all, or part, or your screenname on your server, and doesn't match anyone else on the server at the same time, you will be added as an admin.
So, here are your steps:
1) Connect to your server from within gmod.
2) Open console and enter "rcon_password <your pass here>"
3) Enter "rcon ulx adduser <your name here> superadmin"
4) You should get a response back from the server.
5) It may tell you to run a different command before you can run adduser. If so run this command through rcon ("rcon <command it told you to run>"), then run the add user command again.
6) You should now have access. If not then please read the first topic under Important Topics on this subform "Having trouble with access?", and complete all steps.
7) If you get here and it isn't working, then you should have provided us with some specific information to help you. If you have not then return to step 6 as you have not completed it correctly.