first argument is the material name, get it using uboats' app:
http://forums.eagle.ru/showthread.php?t=159784
2nd argument is the layer, 0 is the diffuse, 2 is spec, most others aren't used
3rd argument is the name of your file, don't need extension if it's .dds or other supported format
4th argument is true if file is retrieved from primary location, false if retrieved from the same directory as description.lua