That should be pretty simple to do.
First of all, I'd connect both optical input and output to the converter (eg. two cables to either A or B bank at MOTU), because although you don't necessarily need analog outputs from ADA8200, ADAT signal from main unit can be used for clocking.
I'd set ADA8200 sync mode to slave from ADAT using sliding switch at its back.
Alternatively you can also use wordclock signal from MOTU for converter syncing, in that case one additional BNC cable connection from MOTU WC output to ADA8200 needs to be made and the switch position will be at WC-IN.
In computer I'd open MOTU audio setup app.. There you need to set working sample rate (44.1 or 48k), clock source to internal (MOTU its the clock master, ADA will follow that as a slave). Also both optical input and output in respective bank (A or B, depending where you connected it) needs to be set to 8ch ADAT optical format.
ADA8200 has lock LED near its power swich, which should lit and indicate, that clocking and synchronization is right.
That's basically it. Inputs from the converter should be available in your DAW.. at channels 17-24 for bank A or 25-32 for bank B.
You've mentioned CueMix FX, but that's only necessary to mess up with for low latency hardware mixes or when you would like to print effects from there (like EQs at inputs tab) to recorded audio. Anyway its control isn't any different for signals from analog inputs right at Traveler (like for mic pres there) or signals coming from external converter.. operation is exactly the same, only those signals from external converter appears there at respective ADAT channels in the mixer.
Michal