This is what the line in your /etc/fstab should look like. Change noauto to auto if you want linux to mount it automaticly. (Might screw up some other stuff, but I'm not sure.) As it is, you will have to open a console and type mount /dev/sda when you want to use it.
/dev/sda /mnt/usb auto defaults,noauto,user 0 0
P.S. found some stuff on the net that implied hfs had to be enabled in the kernel.
"There are other times when you may want to mount a native OS X partition in Linux. An example of why you may want this is if you, say, have a bootable OS X environment on some removal media but want to easily transfer the drive around and/or load it up in PearPC. In this case, you instead would want to enable HFS+ in the kernel. As of 2.6.11, HFS (which is used by older macs) is available experimentally and UFS (which is used on some OS X machines for greater UNIX compatibility) in read-only. Enable Apple Extended HFS file system support."
I found that here....
I checked the man page for mount, and it looks like hfs and hpfs are supported, but by using auto, mount MIGHT not detect it correctly, so try hfs or hpfs for the extended version.
So if auto doesn't work, use hfs or hpfs for the filesystem type in fstab. If that doesn't work, check to see if you have hfs support enabled in the kernel you are running.
P.P.S. You might want to read this too, and that fs type might be hfsplus not hpfs.