Hi, try doing this:
Does the mod have a zip/rar? If yes and if you have it saved in a folder, open GTA Mod Installer and click Install a mod for VC and it should say "Path for Zip/RAR". Then search for the rar/zip and click next, tell me if it works
i tried what you said but it didnt work the mod i wanted the most was the multi rocket launcher but i dont know how to use the image tool