The idea is that you run a separate program that both interfaces with the device and provides a local HTTP server that Snap! can use to make requests to the device. Unlike Snap! itself, these programs have access to anything on your computer, so you have to trust the author of the software! Our web site, snap.berkeley.edu, provides links to drivers for several devices, including, at this writing, the Lego NXT, Finch, Hummingbird, and Parallax S2 robots; the Nintendo Wiimote and Leap Motion sensors, and the Arduino microcomputer. The same server technique can be used for access to third party software libraries, such as the speech synthesis package linked on our web site.
Most of these packages require some expertise to install; the links are to source code repositories. This situation will improve with time.