I just helped another user with a similar error. He had no sound, but the log gave the same error. Try reinstalling FTB to your desktop in its own folder not on your desktop. Follow this
guide exactly to do it. It also might help getting update 45 for Java 6.
Once done if you still get the same error, [INFO] [STDERR] java.io.IOException: Server returned HTTP response code: 503 for URL:
http://s3.amazonaws.com/MinecraftResources/
If you have a vanilla install copy over your resources folder to the pack's resources folder. I would give more info about this process, but I am not that familiar with the Mac folder structure.