We're delighted that you bought an AudioBox USB, but if you need a mixer then you essentially bought 2 audio interfaces. You only need one.
With just the ZED mixer connected to your computer, open up StudioOne and on the start page in the center at the bottom click where it say "Configure Audio Device", on the next page, click where it says Audio Device, it might say "Built In Output", click there and a drop down menu will appear, there you should see your Allen and Heath ZED mixer. Here's a link to a tutorial from v2 that will help you. http://support.presonus.com/entries/58079930-Episode-9-Audio-Setup
You'll need to setup your Inputs and Outputs in Studio One. You should learn how to do this anyway as you'll need to set it up for each new song you do. Go here to watch a video also from v2 that will help you with getting your I/O setup so you can use it in Studio One.
There are more videos in that series to help get you going. All done on the Mac.