Assuming that you already have your diffuse (regular) skin attached...
When your in MED, under the edit tab, goto "Manage Skins".
side note - damn, I just had a spider crawling on my back (little one). dead now

Then goto add new texture, and add the normal mapped texture. It will use the same UV set as the first, so you won't have to modify it or anything

Thats it for the model! But there are still many parameters that can be set by right clicking the "NM.wdl" file under your "resources" tab. These are pretty straight forward...
Try adjusting some things in your level (make sure lighting is present). You can add a dynamic light through code (my.lightrange = x, my.red/green/blue = x;). Setting a sun_color should work as well...
Any more comments/questions, feel free to ask!