The digital output on Airport Express is the mini jack output - it requires a mini-to-toslink adapter (or mini to toslink cable), which to my knowledge are hard to come by unless you buy the Apple/Monster cable. A mini-to-stereo RCA would work for the RR2150.

Unfortunately, the USB port on the AE doesn't support anything other than USB Printers, but [it is rumored that] Apple is working to change this so any USB device can be connected. I'm not a tech expert so the reasons it's so difficult are beyond me. It would be nice to hook up USB devices (ie RR2150) to the network, though.sss