this guide (how to download just cause 2 mods) BUT (make shure you closed just cause 2)
1-go to (http://justcause2mods.com/)
2-Choose your mod and download it
3-go to just cause 2 folder (SteamLibrary-steamapps-common-Just Cause 2)
4-make new folder called (dropzone)
5-put the mod files into the dropzone folder
6-run just cause 2
7-Congratulations