Hi Paul,
No matter what hardware you buy and connect to you Mac you always (ALWAYS) want to go to the developers website and download the latest drivers, etc. As a rule of thumb, never use the ones that come on a disk with your product (including the Io) without checking for newer versions on their website first.
Here's the link to download the latest versions:
http://www.aja.com/html/support_Io_swd.html
Tony!