Please note that with a Mac to use the pre 1.6.4 packs you need to use Java 7 and update the lwjgl to 2.9.0 Look
here for more info on that.
If you've done that and that is not the reason for the error please provide a console log posted to pastebin.com and linked here. To do this go to the Options tab and ensure that 'Show Console?' and 'Reopen launcher after exiting minecraft?' have check marks next to them. Now close and reopen the launcher. Try running Unleashed and after you get the error if you have to close minecraft to get the console back please do. Once the console shows up click on the 'Paste my log to pastebin.com' button. This will copy your log to pastebin.com and give you a link at the bottom of the console. Provide that link to us so we can see your log.