First, press the [shift] key then press the [i/o] button.
Now select the tab for "Output" at the bottom of the display.
You should now be looking at the output assignment screen.
In the left column you will see the source and in the right hand column you will see the output that source will feed. You will see your digital outputs in the last two slots in the right column, and a line attaching them to the box on the left, which is the source for those digital outputs. Scroll down to the box that is attached to the digital output you want to use, then use the data wheel to call up "Stereo" as the source. You have now assigned the stereo bus to feed the digital output.