There are several USB data acquisition devices avaialble for Mac OS.
National Instrumements makes a full line of products.
http://zone.ni.com/devzone/conceptd....701F007A8B40#1
If you only need digital inputs, check out the X-Keys USB Matrix board:
http://www.xkeys.com/
Another to look into is the Phidgets products. I see they now have MacOS support in beta.
http://www.phidgets.com/