Games up to date, It shows up as Installed on the DL Content but not in the Maps installed list. I DL your 1973 C-30 Flat Bed and it is working fine. i do have a couple other mods doing the same or Not showing up in the store just the vehicles add on options
verify your installed game files, I don't know where you downloaded the game but you should be able to find how on google or youtube, that sound like somethings wrong on your end, maybe find the game log too to look for any error while searching those mod name, it will give you a good idea what to look for