Stage Traxx can only change the song volume via remote commands, not the volume of a single track. If you want to change the song volume, you can define how much a single action trigger will change the volume in Settings > Playback Settings > Volume Step Size. Change that to a value that you can work with but keep in mind that the change is immediate. So for example boosting by 12dB with a single trigger might be a bit too much.
Then assign actions for "Increase Volume" and "Decrease Volume" to a midi pedal or select the volume setting for the assignable keys.