I would not use the RCA (CD/Tape) Main outs. Doing so effectively by passes the "Main Control" portion of the mixer. I'm using a 1222FX which is similar.
If you are using the Behringer shipped USB interface...
Get a pair of XLR(Female) to RCA cables.
Live Wire #CPR-305
Get a matched pair of RCA to 1/4 cables.
Live Wire # XRF-110
(they have different lengths)
Attach the XLR's to the Main outs on the rear of the box and connect the RCA's to the Input side of your USB Interface.
Grab a Stereo Channel, 7/8, 9/10 ..etc, connect your 1/4 jacks to the L/R of the stereo channel and then, obviously, connect the RCA's to the associated Output ports on the USB Interface.
This will allow you to take full advantage of the Main Mix Controls, EQ as well as the FX options, although the FX should work either way.
Chap.