The O key shortcut toggles an overlay colour, which will tell you whether you are successfully painting "mask" or not. If set to Erase, or otherwise zeroed as Hal P Anderson says, nothing will happen regardless of the strength of the selected adjustment. The adjustments themselves can be changed before you start painting, during or after.
If you don't see the individual adjustment sliders, note that the brush panel has an expanded and a collapsed mode, controlled by a small arrow at the right hand end of the "Effect" line.
Besides the mask overlay, the adjustment sliders can also be used to set a deliberately extreme adjustment so you can see what you are doing, and then you can tone that down to taste afterward. Dragging on the "pin" to right and left sides respectively, exaggerates or mutes all of the sliders - moves them proportionally further away from, or toward, their zero points.