How does one go about recording 789's when only two speeds are available?
There is a guide on the official website that's titled Advanced Recording. The process explains how to record the discs with the EZ Vinyl Converter application. I recommend accessing the FAQ page where you can see exactly the process. However, to begin the recording procedure, use the Change Speed, insert 78 and start recording.
FAW page: http://www.ionaudio.com/usb-turntable-and-cassette-faq#Advanced Converting