FSDS night lighting not showing in FSX.. need help

Need help.  I have got back into creatign some airport structures for FSX using FSDS v3.5.1... I forget the process for getting night texturing on the structures to display in the game.. After creating a model part and selecting Parts/ Textures and selecting the texture file to use on various surfaces, there is an option to add N ( adds _lm) to the file name.  When I do this it does not work ( i.e. no night lighting effects, even in the FSDS night view window.). If I dont tick the N box and simply choose a file that has an "_lm" counterpart in the FSX/Textures folder, then I can see the night lighting effects within the FSDS window but after compiling it, in the game itself it just shows up black.

Can someone advise.. What am I forgetting to do?

I have the Airport Wizard and thought that was causing it ( I know if that software is left on, it causes conflicts), but that is unticked in FSX and within the software itself, the airport in question is set at "undeveloped" and applied. so I am clearly forgetting something else rather basic.