There is a master aux send knob. turn that up. If that doesnt fix the problem, we need to troubleshoot.
Connect aux send 1 to the input of the lexicon
connect the output of the lexicon to the aux returns
send signal through a channel and turn up aux send 1 on that channel. Are you getting level at the lexicon?
If not, find the aux send master and turn it up. If so, good.
Plug an output from the lexicon into a channel on the board. Turn up the fader, do you hear the lexicon?
If so, plug it back into the aux input. If not, the lexicon or the cable is broken.
Sometimes there is a volume control for the aux return, make sure that is up
Sometimes there is a button that routes the aux input to the main mix or headphone mix or someplace elses. make sure that is set correctly.
Having said all that, the aux return is never sent to any of the direct outputs. If you are trying to hear the effects in cubase, it wont happen using the aux returns.