I figured it out, put all the .vox files in the custom folder and change the custom.xml to the custom.xml you downloaded, if it doesnt work let me know ill try to help
Also I'm not sure if the download is suppose to contain a .vox file or not but when i downloaded it i only had the default game vehicles .vox files and a .xml file. When I download other maps I get a .vox for the map, I'm not sure if im just doing something wrong or not