i've done it before, but can't get it now...

i've taken a source image of a brick wall, and made two versions, one color, one b+w for the bump map. i selected the color one for the color attribute, and the b+w for the bump mapping attribute. when i apply it all i can see is the color applied, but no bump.
it's a lambert applied to a nurb square, if that makes any difference. should i make two lamberts, one for color, one for bump or something?

if anyone knows what i'm doing wrong, or knows of a handy bump mapping tutorial i'd appreciate it. thanks.