Hi,
First time using this brush so maybe I’m using it incorrectly. The pop up tells me Ctrl Shift and click. Well when I press the keys there is no way for me to click the mouse as I’m holding the left mouse button down in order to draw the shape out so that makes no sense to me.
Please help.
Thanks
Richard
Hold down Ctrl+Shift then left mouse button to draw the rectangle. After you have the rectangle drawn out, you can let go of Ctrl+Shift (the rectangle will still be there). At that point, you can hold down Spacebar to move it, let go of Spacebar to resize it again or press Spacebar again if you want to move it some more. Once you are happy with the size and placement, you can let go of the left mouse button to complete.
Thanks so much! I appreciate your help! 
No problem RAMWolf. 
I should add that the reason you were seeing that pop up is that you chose the brush right out of the brush palette and ZBrush is just reminding you that you need to press Ctrl+Shift to use it. Any selection, clip or trim brush uses Ctrl+Shift. So to eliminate seeing that pop up, press and hold Ctrl+Shift first then click the Brush Icon and select which one of those brushes you want to use.
Oh, that’s a useful tip too! Thank you!