First thing's first, select the pencil in the slicing settings menu a the upper right hand corner and click "Custom". (first photo)

Next click the hamburger menu and select "Show All Settings". This will become useful later on as you get more familiar with 3D printing and slicing software. (second photo)

Now, let's go over some of what you see on the screen (refer to third photo):

At the top you'll see the two colors used to distinguish the two extruders for a dual extrusion machine. These are custom made material profiles that say PLA, but the material type and color in real life doesn't matter. This is just a tool in the software to show what parts will print with which extruder.

At the left is where you can change which extruder a part prints with. Simply select the part, then select the color associated with the extruder you want to print the part. Blue for Extruder 1, grey for Extruder 2.

Your main build area is represented in the center. Our Quick Start project files have our dual extrusion SeeMeCNC batarang file (edited from this file) pre-loaded, but you can easily delete these files and import whatever you want.

This circle "shadow" is indicating the approximate location and size of the purge tower that has to be used when printing in multiple materials. You'll see what it actually looks like when you slice your files and preview it.