IF you are using the Debain Wheezy image from gwolf this will not work as the kernel is monolithic and won't accept loading modules

You need at least the 3.1.* to make it work properly. My suggestion is to install xbian for the moment. Or try to flash the xbian boot partition, with the wheezy rootfs partition.

For better results please disable journaling from your hard drive

http://julipedia.meroh.net/2007/04/how-to-disable-journaling-on-hfs-volume.html

Install hfs packages

apt-get install hfsprogs hfsutils hfsplus

Find your device UUID

blkid /dev/sda*

/dev/sda1: LABEL="EFI" UUID="5F66-17ED" TYPE="vfat" 
/dev/sda2: UUID="28e97702-9515-362a-992d-d5ae5cd99783" LABEL="Media" TYPE="hfsplus" 

In this case our drive is 28e97702-9515-362a-992d-d5ae5cd99783

Open fstab

vi /etc/fstab

Add the following

# HFS Hard drive (Media)
UUID=28e97702-9515-362a-992d-d5ae5cd99783       /mnt/Media      hfsplus defaults,rw     0       0

If you want to replicate the mount for your plex root also add the folowing

# Replicate the mount for Plex
/mnt/Media       /chroots/wheezy-armel/mnt/Media       none bind 0 0

Mount the drive

mkdir /mnt/Media
mount /mnt/Media
mount /chroots/wheezy-armel/mnt/Media/