adding layer mask problem, (CS5 mac)

Hi, I'm working with CS5 mac 10.5.8, working with raw files .CR2

I'm trying to add a adjustment layer to an image with layer mask as follows: add adjustment layer, option click layer mask, select brush tool (100% Opacity) make sure the white for the mask is selected and paint with my brush, but nothing happens.

I tried the same thing but instead of the adjustment layer I duplicated my background then option clicked the layer mask, and it works.

the only difference I noticed between the two is that when adding the layer mask to the adjustment layer, the mask that appears in the little icon is grey. however when adding the layer mask to the actual duplicated background, the icon is black (which is how I think it should be)

what am i doing wrong? please help! thank you!