Click the 
icon on the toolbar.
This will open the Output Configuration
dialog box.
Make sure that your main soundcard is selected in the "ON-AIR" output device and that Enabled option is selected.
Now select the Rear channels option in the Use on-air device for cue: options.
Close the Output configuration dialog box.
To test, play your favorite Ots file.
Select the Mixer
by clicking twice on the 
button on the toolbar.
Click the Cue button. You will notice the cue channel output through the rear channels.
Note: If you have received the "Failed to allocate audio device. Close other audio programs and start again" error message, this could be because you are trying to enable the same soundcard in the both the "ON-AIR" and "CUE" output options. You can not allocate the same device in both places. The Cue channel selection for multi-channel ASIO soundcards is located at the bottom of the 'Primary audio output' section of the dialog box.
Tip: If you notice
cross-talk (cue channel in the main channel and vice versa) you may need
to optimize the Air and Cue cross-talk.
If you notice any warbles and/or pops you may need to calibrate
your soundcard.
You may notice the ON-AIR signal through the cue channel. For instructions
on how to remove the "ON-AIR" signal from the cue channel click
here.
How to optimize Air & Cue cross talk
How to remove "ON-AIR" signal from the cue channel
How to set up your soundcard (main channel)
Output Configuration dialog box