Insert the files and folders you want people to download into garrysmod/data/ulx/downloads.txt, one file or folder per line.
If a folder is added, all the files inside the folder will be recursively added. Linux path notation ('/' not '\') is preferred.
Add a file from the garrysmod root. IE, if you have /garrysmod/sound/mycoolsounds/awesome.mp3, you'd add the following line:
sound/mycoolsounds/awesome.mp3
Never, ever have /addons/<mod> in addForceDownload! If you have something like addons/mycoolsoundpack/sound/mycoolsounds/awesome.mp3, you'd use the same line as above.
If you're using sv_downloadurl (if you don't know what it is, ignore this bit) then you'll need to stick what you're forcing to download in with the rest of the sv_downloadurl resources, using the same folder format.