To change that and assign a mixer track to a channel, you need to go into channel settings (just left click on the channel of interest) and select a mixer track. You actually do that by changing the value in the FX box (upper right). By default, when you add a channel, it's "--", which means master mixer track. Once you've done that and go to the mixer, you'll see that your channel has now its own mixer track and you'll be able to apply effects (up to 8) to that channel/track.
The video below kinda explains that.