I will just copy and edit an answer to a different question but I believe it still applies here:
This problem is probably due to kext signing introduced with OS X 10.10 Yosemite. With it you can't run any unsigned drivers and old one is probably unsigned.
You might try to disable kext signing if you want to use the driver. Make sure you absolutely have to do it and you don't have other options since this change is system wide and system is becoming less secure after that.
sudo nvram boot-args="kext-dev-mode=1"
And reboot. Then try and report if driver is working.
To go back to kext signing put this in Terminal:
sudo nvram boot-args="kext-dev-mode=0"
P.S. I have become interested in the DAC and now I see it has Optical Input. So other workaround might be just using miniToslink to Toslink (or just normal Toslink cable if your Mac is a Mac Pro)