Make sure you have followed all of the steps featured in this video:
You need:
A reference to the car in 'dlclist.xml' to load it in game. You can use Ctrl+F3 to search for stuff in OpenIV.
The basic format in 'dlclist.xml' is like this:
<Item>dlcpacks:\*dlcpack car folder name here*\</Item>
Example:
<item>dlcpacks:\challenger16\</item>
<item>dlcpacks:\gto2\</item>
<Item>dlcpacks:\viper\</Item>
<Item>dlcpacks:\mustang65\</Item>
The 'dlcpack car folder name here' corresponds to the name of the folder in dlcpacks
Example:
mods\update\x64\dlcpacks\16challenger
mods\update\x64\dlcpacks\gto2
mods\update\x64\dlcpacks\viper
mods\update\x64\dlcpacks\mustang65
Add-on cars come all pre-packaged & working so I can only think something is wrong in your dlclist.xml.