This is a short little tutorial on decals. Simple black and white image, what you need to do is add a channel for it. An alpha channel is the black , white, grey areas translated to gradients of transparency in the picture. The image below will be the diffuse map saved as a TGA or TIF with the alpha channel we added. Ok, on to the MAX stuff.
First step is to load the composite type map as the diffuse map.
You'll see 2 slots. Here is one more image with Cellular being used in that top slot for the composite material.