Can't add colors to mixer palette

Hi. I just bought Painter Essentials 5 for Mac. I also own Corel Painter 12, but I found that I get overwhelmed by all the options in Painter 12 so I bought Essentials because i needed something simpler. I am using Essentials with a Wacom Intuos 3 tablet.

Anyway, in Essentials, I am unable to add colors to the Mixer palette. I have no idea why I'm  having problems. I'll choose a color, then choose the brush in the Mixer palette but when I try to paint with the brush in the Mixer palette, nothing happens.

I tried adding colors to the Mixer palette in Corel Painter 12 and that worked.

Does anyone have any idea what's going wrong in Essentials?