The Zed 12FX is a lovely little mixer and it does have a USB interface so you don't need an external sound card (unless you want more than 2 channels in/out at some point in the future).
You need to instruct your system to use the A&H USB interface in at least three places. First off, you need to tell the mixer what to send to the USB. Unlike a lot of less complete mixers, on the A&H you have a choice of using the main L/R outs, Aux 3/4, or Aux 1/2. I might use one of the Aux sends because the USB returns go to a channel and can be routed to the main outs that way--but I'd have to think about what's best.
Anyway, once you make sure the proper part of mixer is sending to the USB, you have to go to your computer control panel/Sounds and Audio Devices section and make sure that "USB Device" is selected on the record side there. (This assumes a PC--if you have Apple, somebody else will have to help.)
Finally, in Sonar, you have to go to the input section and make sure USB Audio device is the selected driver and source there.
The A&H manual gives pretty detailed instructions starting at about page 24.
Bob