All of which makes it very clear that the real challenge is not so much knowing how to do it but explaining it to other people.
So Bob, you probably have almost everything right except that we do not manage to explain well what should be done.
The latest development build from Arno is from July 6th. I haven't the faintest whether this is 1.2 or not because I do not know where to look for the build no..
I am glad Doug hops in to help us out (although he thinks I suffer from a lack of sleep

), but we are both saying the same thing but in other words. Maybe Doug manages to explain better than I can.
I was not referring to obvious questions, Bob, I was referring to all of those most common mistakes I made myself (and still do once in a while and then I have to look for my own thread on common mistakes to check what I did wrong this time

).
So I give it another try in explaining:
You made a Google Sketchup building and you made a rectangle where the window should come. Then you added a texture to that window. You then saved your work and exported it as a dae file.
You then opened ModelconverterX, went to the material editor and found your texture for the window as xxxxxxx.jpg.
Now use a paint program like Gimp or paint or paintnet and open the xxxxxx.jpg. Darken your texture or paint it yellow (and some parts black

) and save it once again but this time as xxxxxxx_LM.jpg in your GSU object folder where all your other GSU object textures are found.
Go to MCX once again and make sure your texture path is set to the folder where you keep your GSU textures (if you had another texture path you would have to close and reload MCX).
Find your window.jpg, click on it and add your window_LM.jpg or whatever you called it) to the night texture map.
You should now be able to see your night texture if you click on "render night".
Your day and night textures still could have the name "texture.x.jpg" and "texture.x_LM.jpg" but you might want to change that into something you know exactly what it is, like window1.jpg and window1_LM.jpg.
Now that you have done that, and I sure hope that you now do see your night texture, you use the mass converter and change the texture file format to dds or bmp. It will make your model go untextured again if you do not enter the new file name plus its new extension in the material editor 'diffuse texture; and 'night map texture'.
The only things you have to do afterwards are to
1. export your model
2. copy your .dds or .bmp files to the texture folder of your model.
Gee, the more you try to explain something simple, once you understand it yourself, the harder it gets to convey it simply.
I need a drink! Cheers and take one yourself, it might come in handy to fight off your frustration.
Roby
PS: No, wait with that drink, it is too early for you, it is almost 7PM here but rather early for where you live.