Just downloaded 1.5.1r2 on my 10.7.5 machine, and it ran ok. Uploaded some sketches to an Uno, but I don't have a Due. In case it makes a difference, java -version reports:
java version "1.6.0_37"
Java(TM) SE Runtime Environment (build 1.6.0_37-b06-434-11M3909)
Java HotSpot(TM) 64-Bit Server VM (build 20.12-b01-434, mixed mode)
Thanks Nick, that appears to have done it. The hardware folder contained a attiny45_85 folder. I just moved it out of the sketches folder and now it will run.
Any ( publishable ) suggestions of what to do with the folder?