By "SUM" your audio, you mean entering a number to pan one of your tracks, correct? After fussing with the new MC6 Audio Mixer for an hour, I discovered that if I click the new round slider button, and hit "+ 100" (or "+" and a number) and then "ENTER" it will pan R100. For panning left, I click the round slider button, hit "-" and a number, then "ENTER" and it will go L/whatever number I enter. If I click the new round slider button and hit "0" and then "ENTER" it will go to "MID."
Not sure if this answers the question, but hope it helps someone. I miss the old slider.