If you want to stick to the Moon 100D you have three options:
1) USB from iMac
2) Toslink optical from iMac
3) Add a streamer like Airport express or Squeezebox
(1) and (2) should be similar in this case as the 100D's USB implementation is Class 1, so limited at 16/48. So depending if you want to place the DAC far away from your Mac, then you should go with optical. You should only use short-ish USB cable.
(3) gives you more flexibility on placement but ofcourse you have to buy an additional device, if you use Sqeezebox then you have to run squeeze server software on your mac and use the Touch to "pull" music from your mac. There is a dedicated control software that works on your ipod/iphone or ipad. Airport express integrates completely with iTunes and Airplay. In both cases you can take digital output from streamer to your 100D. Please note that Airport express also limited to 48k, but Squeezebox can do Higher resolution.
Another solution is to sell the 100D and buy a newer DAC with asynchronous USB Class 2 input. Where you can run USB cables up to 5m.