Because ulx forums typically has people who are new to garry's mod/addons/lua, I suggest telling people where to specifically add the file. I know that it goes in lua/autorun, but others may not, you may even have to go as far as telling them go to steam/steamapps/garrysmod/garrysmod, ect, ect. Garry kinda made the newer gmod community players kinda dumb with all this workshop stuff.
Also, some more questions that maybe should be answered here on your original post. Can only admins set buildmode, or can anyone set themselves to buildmode? Does this require ulx, or use ulx in any way? Maybe a few more instructions on how to set things up once this is installed?