QuickTime in Mac OS 10.9 does not always play back HERO3+ files properly. Apple is currently looking into this issue. The workaround would be to download QuickTime 7 from Apple's website, as the older version is still available.
QuickTime will play source files from the HERO3+, and it will also play files converted with GoPro Studio.
Another option is to download VLC Media Player. This will play source files from the HERO3+, but it will not play files converted with GoPro Studio since it does not support the CFHD codec.