It appears some people seem to be having issues with this, and I did as well... Here's some things to take note of.
1. By default it is Left Shift. I'm not sure about other schemes, but this is for WASD.
2. I do not believe you can remove a modifier key; you have to change it to something else to have another function use [modifier key]
How do you use it?
1. Hold [modifier key]
2. Press [example: attack key]
How do you set it?
1. Click on a control for a function [example: attack] like normal.
2. Hold down [modifier key]
3. Press [new attack key]