Make sure it has a description.txt file and a server.cs file. The description.txt file should read like this when you open it:Title: Jeep On Steroids
Author: Muslim
(Description of Jeep On Steroids here)
It could also be that the namecheck.txt (if you have one) does not match the name of the vehicle. For instance, if the Jeep on Steroids add-on is called, Vehicle_JeepEdit, the namecheck.txt could be saying Vehicle_JeepEdiy, so make sure the namecheck.txt (if you have one) matches the name of the add-on. Also, there are no spaces allowed in an add-on name, so it would have to say Vehicle_JeepOnSteroids instead of Vehicle_Jeep On Steroids.
Hope that helps!