The method to assign channels to mixes using the surface is to hold the Assign button, and then press the Mix button of each channel (or mix). However, one has to look at the LCD to see if it is ON or not. This means that you are looking in one place, and pressing in a different one.
To provide better visual feedback, while the Assign button is held, the Mix buttons could light up to reflect the assignment.
This feedback could also be applied to the Pre/Post button.