The drivers are on the DVD.
Go to your device manager, you should see a RIYA device under COM ports. If you don't go back to your control panel and click the add new hardware icon after you have plugged the DAC in. It should scan and find a new device, I think it said Laser control or something like that...
After that tell it to install from a specific location, then choose the driver to install (or something similar, can't remember the exact prompts...) You should get a file selection box, navigate to the RIYA Driver folder on the CD and then pick whichever OS your running. Once you pick the right OS folder it should pick the file on it's own. Then click OK and let it go.
Somehow I got extremely lucky, I just plugged the DAC in and told windows to find a driver and it did. That in itself is miraculous, but what makes it better is that I'm running XP64. Nobody makes drivers for XP64, I was pleasantly surprised to find that it will work.