They look like this: (imagine a picture here). They have blue ends with a twisted grey cable and are about 30cm long.
It turns out that they're Prolific devices, and you can get drivers for them from here. On a Mac, they present as /dev/tty.usbserial, and the best way to talk to them afterwards is with minicom from MacPorts.
Information in this area is meant for use by CSCF staff and is not official documentation, but anybody who is interested is welcome to use it if they find it useful.