1) When using - rcon ulx adduser "<some_name>" superadmin - ... <somename> must be connected to server/in game. Otherwise, see help for adduserid
2) When using - ulx adduser "<some_name>" superadmin - from in game, person running command must have already be added to superadmin from console (or default gmod users configuration text file as recommended in many of our ulx_readme.txt links.
98.something% of all mods that look for admin and superadmin rely on that file. Even ulx uses that file by default, using rcon is just bonus.
See also Our Fine FAQs.
How do I add a user? - Since you seem to be having trouble right off the start, including with other admin mods, I'd bet its portion two..the GM10 /cfg/users.txt file (Again, thats not even a ULX file, its Gmod..we (and most other admin check) import it.
Having trouble with access?If you didn't download ulx and ULib from any ulyssesmod.net site (SVN, or www. download page), then we recommend you remove them, then re-install from our site.