How do I put a DAC to good use?

I have a Pass Labs DAC 1 that is not being used. I thought about selling it, but I think it's one of those components that's probably to hard to sell. So I thought why not stream music through it. I have a Mac with iTunes and a Airport Express, but the AE only has USB out. How do I go from a USB to any of the DAC 1 inputs, be it XLR, COAX or Optical. Is their a component that can go inbetween these two? Thanks for all your input.
I thought the Airport Express had an optical out? I swear people said they were streaming to AXs and using optical...
This isn't an answer to your question but a clarification. The USB port on the Airport Express isn't an output, it serves only as a connection for a shared network printer.

The output of the Express is a mini-optical jack. You can stream wirelessly from the Mac to the Airport Express, then go directly from it to the DAC 1 by using an optical cable. The direct USB connection you're after is supposed to offer higher quality but you could try the setup just with a cable and the adapter for the mini-optical port. I've been happy with a glass cable I bought from a member on this site, Captnstarstripe.
Gmood1: Thanks. This might just be what I am looking for.
Edesilva: My AX only has a 1/8" stereo mini out.
Sfar: Not sure if your statement about the USB port is correct. Gmood1 sent a link to "trends audio UD-10" that seems to convert the USB to other connections that go directly into the DAC. I'll have to look into this.
Thanks everyone.