This might be too late for the OP, but maybe it will help someone else.

You CAN shrink an hfs+ partition on an MBR drive, you just can't expand it at this point, at least as far as I know.

To shrink the partition, start by disabling journaling

sudo -s
diskutil disableJournal diskXsX

obviously replace X with your target disk and partition, then use a linux liveCD such as

Parted Magic

or Ubuntu or anything that has gparted or parted available, to shrink the partition, reboot and re-enable journaling

sudo -s
diskutil enableJournal diskXsX

It goes without saying that you should make a backup before playing around with your filesystems.