The X bar moves through the matrices left to right, and the Y bar moves up and down.
Right click the box to the right of the bar, allows you to select what controls it, in your case it will CC something, if you pick CC1 here and then move your mod wheel on your keyboard you will see the dot in the bar move. You should also see the matrix change. You can assign any controller or other MIDI event to control this, but generally CC's are the best (some users use keyswitches).
The value of CC to pick a particular cell can be adjusted by grabbing the dividing line and moving it