If your using gmod9 dedicated server then you need ULib v 1.2 and ULX v 2.21. Those are the latest for gmod9.
First download and install ULib and the ULX. If you need help installing then read the readme's in the zips.
When you have installed them open up users.ini wich is somewhere in the Lua/Ulib folder. In ther, type your SteamID or whatever you want, so that u can be admin on your server.
Now ULX should work fine. In the server console it should load the ULib and ULX.
If it does not, check the lua/init folder. In there should be this file: ulib_init.lua
That's the file loading ulib at server startup. But I think you aleready know that..
And if it still does not work check if lua/ulib/modules/ulx_init.lua is there. If it is not you haven't installed ulx right and you should do it again following the steps in the readme.
I hope this makes ulx work for your server.