Thanks for trying this out on OS X. I don't own an OS X operating system so I can only help you with theory and experience from the port to OpenSuse.
The plugin code uses a full path to locate the avr-g++. For windows and Linux this means that adding avr-g++ to the path doesn't help. I guess this will be the same on OS X. In windows avr-g++ comes packaged with the IDE. In Linux it is not. Is avr-g++ delivered as part of the IDE in OS X?
Because avr-g++ is delivered differently per OS a part of the porting includes having setting different paths.
You can however set the paths yourself. As an experienced eclipse user that should be a piece of cake.
To set the paths yourself, as a work around, you do as follows:
Locate avr-g++, GNU make, the AVR header files and AVR dude on your system. (You don't need atmel part description files)
open eclipse and go to windows->preferences->arduino->paths. You should see something like this:
In your situation there will be errors. Change the paths.
Try again and hope there are no other incompatibilities.
1) you will have to fix all the errors before you can save the preferences.
2) Do not open any other tabs in the preferences as this may invalidate the changes you do.
3) Best is to do the change, save, reopen the preferences, validate whether the change is done, if ok do cancel else redo.
If it works it would be nice if you told me what you had to change. This way I can modify the plugin to set these values correct.
Even better was if you changed the plugin yourself (with my help off-course).
Lots of good luck