Here's a constructor of my Checkpoint class used in Transportation:
public Checkpoint(Vector3 Position, float Radius, System.Drawing.Color Color)
{
_fRadius = Radius;
_vPosition = Position;
handle = Rage.Native.NativeFunction.CallByName<uint>(
"CREATE_CHECKPOINT", 5, //5 is a type ID, check natives documentation @ http://www.dev-c.com/nativedb/
Position.X, Position.Y, Position.Z - 7.0f,
Position.X, Position.Y, Position.Z,
Radius, (int)Color.R, (int)Color.G, (int)Color.B,
100, 0);
_bExists = true;
}