As usual, the internet has an answer to everything – http://jclark.org/weblog/2005/05/24/ubuntumount/. There was one thing I did differently to get going so I figured I’d note it here, in case I need to do it again.
In order to figure out where my disk was so that I could use parted to examine it I used fdisk -l.
root@subfocus:~# fdisk -l
... the info about my other drives, then ...
Disk /dev/sdc: 320.0 GB, 320072933376 bytes
255 heads, 63 sectors/track, 38913 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x00000000
Disk /dev/sdc doesn't contain a valid partition table
So that identified the hard disk as being on /dev/sdc (this was a usb drive).
Then I used parted as he did in the article,
GNU Parted 1.7.1
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) select /dev/sdc
Disk /dev/sdc: 320GB
Sector size (logical/physical): 512B/512B
Partition Table: mac
Number Start End Size File system Name Flags
1 0.51kB 32.8kB 32.3kB Apple
3 134MB 320GB 320GB hfs+ Apple_HFS_Untitled_1
Note the select to switch to the correct hard disk. You can then use that Number to identify which partition to mount. I therefore needed to mount /dev/sdc3.
root@subfocus:~# mount -t hfsplus /dev/sdc3 /mnt/macosx
As Mr Clark mentions, you should remember to unmount the hard disk,
root@subfocus:~# umount /dev/sdc3