vehicles.PlayerSpawn() returning false

I have a Sandbox derived gamemode, with the following in a sent’s Use function, which is taken from the Wiki:

[lua]vehicles.PlayerSpawn(ply, “jeep”, ply:GetPos() + ply:GetAngles():Forward() * 64, ply:GetAngles():Up())[/lua]

For some reason the function is returning false, with no errors. I’ve also tried adding the following from this Wiki page: http://wiki.garrysmod.com/?title=Vehicles.AddVehicle, which resulted in an error of vehicles.AddVehicle not being a function.

Could anyone shed some light on this please?