bale 2 options in putting distortion is:
first, to use the drive knob, select which drive you prefer, you can directly change the first setting of the drive module by tuning the 1st knob which will change the gain settings.
2nd, to use the edit button found below the play button. use the module button to select the drive module (in between the iso/comp and the ZNR). then by selecting the effects button, you can select the which drive type you want (acoustic, higain, dist, OD, clean, fuzz. etc.) then each effect/drive type has 3 adjustable settings, for the drive it has gain, tone and level assigned to the 3 knobs beside the play/edit button.
just remember if you're satisfied with the settings you've made, don't forget to press the store button (twice) beside the effects button.