HFS+ is the files system used on many Apple Macintosh computers by Mac OS. You can mount this filesystem in Ubuntu with read only access by default. If you need read/write access then you have to disable journaling with OS X before you can continue.

In OS X, open a terminal and do

sudo diskutil disableJournal volumeName

There was a bug in certain versions of OS X that would allow this command to complete successfully, but still not disable journaling. In order to fix this issue, you may need to enable journaling first with the command:

sudo diskutil enableJournal volumeName

Disabling journaling on your main OS X partition is not recommended however as journaling is an important feature of any filesystem that can prevent damage and data loss: http://en.wikipedia.org/wiki/Journaling_file_system

Other information which may be useful can be found here: http://en.gentoo-wiki.com/wiki/HOWTO_hfsplus