Okay guys. Everything works now. Here's how make it work:
1.) start OpenIV then go into edit mode and update "update.rpf". (You'll see a big-ass green text on your open iv platform)
*** It is recommended that you make a backup of 'update.rpf' before updating it; in case you had modified files in it because this process will over-write it. ***
2.) remove "mpgunrunning.asi" from your root folder if you had it there.
3.) download this mod: https://www.gta5-mods.com/scripts/mplowrider2-in-sp
4.) unzip file and put "mpsmuggler.asi" in root folder, so that your vehicles won't disappear.
5.) If you've got add-on cars, please wait for the latest gameconfig file.
You're all set.