How would I add cabin lights to a plane? I have a Gulfstream 4 and at night there are no cabin lights. Can I add cabin lights by adding a line or two in the aircraft cfg file? Or would that have to be painted in using a paint program?

Take a look at some of the default planes, like the 737. You'll see files like xxx_l.bmp and xxx_t.bmp. They cover the same area, but one (the _l file) is the "night mask." If you open them up, you'll see a bunch of windows, and probably a "searchlight beam" or 2 which illuminate things like the tail art. For a given plane/model, all of the "night masks" are generally the same file. So, if you have one Gufstream 4 which does work at night, simply copy the missing _l files over to all the ones that don't.It's a little more complicated if you have no _l file to work with. There, you can just paint the windows some light color to get them to show, but for the tail art you need to play with the alpha channel. For that, look around for some painting tutorials which explain how these work.

Thanks, I thought I could just add a line in lights section of the aircraft cfg file to get the cabin lights. I don't have any of the paint programs so I guess I will just leave it alone.

You can download DXTBmp here. That will allow you to open the files that are in Extended Bitmap format, like most aircraft textures. Then you can use the Paintbrush program that comes with Windows to edit simple things. It's not fancy, but it should be adequate.