Yes, we all know how to hook up and route ADAT, but your ultimate goal is unclear.
I want to help, but I am not sure what you want to do. I think you are trying to find a way to go into your mac via FW, without having to buy an IF-FW/DM card for your DM4800. Understandable, since they are pretty expensive, although you do have a 4800 which is also pretty expensive. The IF-FW/DM card is the way to go, by far. Everything you want to do can be accomplished with an IF-FW/DM card.
If you are simply trying to plug in mics and send them into your mac, then just plug them into channels 1-8 on your DM, assign them to mic/line channels 1-8 on the DM's input screen, then assign those channels to your DM's ADAT outs, 1-8, on the DM's output page for ADAT. You dont need a seperate card for 8 channels of ADAT, it's already built into the back of the mixer. Plug your ADAT cable into the back of your DM, into the ADAT out, and into the ADAT in on your MOTU, I'll just assume it's a 2408mkIII. Now, on the MOTU, set up that unit to accept a clock signal from the ADAT cable, and make sure it matches the DM's sample rate. Now plug the FW cable output of the MOTU into your FW jack on your Mac. Open your DAW and assign those channels to the DAW's channel inputs. Make sure your DAW is also set up to accept the same sample rate for recording.
You could go ADAT out to your octopre, but whats the point? Unless you need 8 Digital channels from the DM converted to analog. But you already have digital to analog conversion with your DM.
You could go Octopre- ADAT out- into your DM, with mics plugged into your Octopre, that would work as an 8 channel external preamp for you. But your best bet for this is probably to go from the Octopre's analog outs, using 8 TRS 1/4" plugs to the DM's analog line ins or analog assign returns, but you will be limited to four channels only on the assign returs. Let the DM convert those inputs to digital at 44.1khz or 48khz, (to be able to use all 8 channels) then go out of the DM via the ADAT out to your Motu's ADAT in, set it up as explained earlier, and then out of that via firewire to your mac.
Assign those signals to your daw channel inputs.
You can come back out of your DAW via FW into your MOTU, then out of the MOTU via ADAT and back into the DM as well, for return channels if you like.
Or just save up and buy the IF-FW/DM. Enjoy the routing fun!