How do I do this in Photoshop CS5?

How do I make Photoshop show the paint brush in the upper left corner for the mixer brush tool? You know, the image that looks like a paintbrush with a handle? It's the gray inset box that overlays on the doent. Someone told me to click on the "toggle the bristle brush preview" icon (the little paintbrush with the eye by it at the bottom of the brush panel) but it does nothing and when I mouse over it, there is a circle with a line through it. Why won't mine show it?! So frustrating!