Assuming you've got ELS v8, there should be a folder called ELS in your main GTA directory. There should be another folder caled ELS in the download (pretty sure that's how bxbugs does it) and in that folder is most likely a file named after the vehicle, probably ending on .ini (like police2.ini). Move that file to the ELS folder in your GTA directory, then go into the slotcontrol file already there, find the vehicle you replaced with the downloaded vehicle and change default_config into the name of the file that came with the download. Make sure status is set to on (default) and you're ready to go.