Good news for those suffering the well know flash issues with NXT-G on the Mac. LEGO education have just release a patch that will fix those issues.
*IMPORTANT - This is for the education version of the software, not the Retail*
From the downloads page -
If you have NXT Education software version 2.0 please use this update to correct all known technical issues with the software including Flash. This update does not include any new features. Download the appropriate file and double click it to install. Follow the onscreen instructions to complete installation. LEGO MINDSTORMS education version 2.0 need to be installed prior to installing the update.
You can download the patch from here (look for the 2.1f5 patch)