I've used CE through Audition and I'm pretty sure all the versions of CE from 1.5 through to audition 2.0 have this setting in the exact same place.
If you haven't figure it out yet you should open up CE and from the options menu up top click edit/audio hardware setup. As long as your getting signal going in the settings here are the most common issue. At the bottom you will have a selection for default input and output, check here and make sure you have your Lexicon interface setup as your input device. If you can't select if here and get things going check your input/output ports, (right above the default input/output selector) and make sure your lexicon interface is showing up in the box there, (the individual tracks from your interface should show up in the Mono section. If it's not there then you have a driver issue.
Now if they are showing up you should check the actual track your trying to record on. Right below the track number is the input selector. Click that, go to the mono drop down and see if you can choose the channel on your Lexicon that you are using and give it another go. Coincidentally, you can also access the audio hardware setup screen from the input selector drop down menu under any track.
The same thing applies to the edit view of CE as far as choosing the right audio input to source from. Back before Audition 2.0 you could easily record background audio, (like web content, etc.) just by setting up the input and the windows mixer properly. It was very handy for grabbing audio from random sources.
To do something you've never done, you have to do something you never did.