-Most probably you have a corrupted file. Try downloading the mod again and try it out. If the problem persists, you should report it to us so we can fix it later on.
-Ensure you do not install mods in your GTA 4 root directory as they cannot be executed if added there. Add them to GTA 4 User Files > MODS Folder instead.
-It is highly suggested to use a save file as old as your mods. If you have modified files in your GTA 4 User Files, the saves with those modifications will not work properly with unmodified save files.
-Some mods do not support multi-player and they can only be activated in single player mode. Others require certain DLCs to work. Make sure your game is updated with the latest DLCs before trying out any mod.
-If you try activating a mod that has updated files, it will not work because the updated files are not in your game directory. You need to copy over all of them into GTA 4 User Files > MODS Folder with the help of SparkIV or OpenIV.
-If it's still not working, open up your mods.img with SparkIV and remove the modified files from the modded version of GTA 4 User Files > MODS Folder. You can then try activating your mod again. This will overwrite the original files in their respective folders so make a backup first if you are not sure.