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 type:

diskutil list

You will obtain an output similar to the following:

/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *256.1 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            230.0 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
   4:       Microsoft Basic Data                         25.2 GB    disk0s4

Note down the identifier of your OS X partition. It is of type "Apple_HFS" and usually named "Macintosh HD". In our example, it is disk0s2.

To disable journaling, type the following in the terminal:

sudo diskutil disableJournal volumeName

Where volumeName is the previously discovered identifier for OS X partition (disk0s2 in our example).

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/Hfsplus