To load in a map for diffuse lighting, you need to click the little "lighting bolt" icon next to the diffuse channel, in the effects & lighting tab of the advanced material editor. You will then land in the function editor. Click the image map node on the left side of the window, and connect it to the diffuse output ' delete the constant node attached to the diffuse output first), then load your image map.
I think you need in finite for this, but I'm not sure.*
The result will be the diffuse lighting will appear on the lighter & white parts of the diffuse map, Darker and black parts will be illuminated by the ambient lighitng.
To avoid having your object using more than 100% lighting ( diffuse+ambient), load the same map in the ambient slot, but invert the image. If you don't do this, the object using this material might become luminous, which can be a problem when using advanvced lighting models.