For clarification purposes, please answer this question: Are you trying to create your own mods for GTA IV, or are you trying to download and install mods already made (like the ones found on this website) to run on GTA IV? Obviously the former is much harder to do and takes time and effort, not to mention either coding, 3D modeling, and/or texture editing experience to accomplish, while the latter is relatively simple. Based on your description, I'm not quite sure which you are talking about, because you simply say "the mod."
Please let me know so I can figure out how to assist you further. Thanks, and have fun modding! :)