I have an old MacBook running OS 10.6.8 which I want to recycle, but its internal SuperDrive doesn't work, so I can't use the install disk to do it. I tried plugging in Apple's external USB SuperDrive which I bought when I upgraded to a MacBook Pro which has no internal SuperDrive, but the external SuperDrive doesn't accept the disc; it's as if the old MacBook doesn't recognize the external drive. I'm thus at a loss as to how to proceed to wipe the hard disk.
You should try to create a bootable USB stick with some kind of recovery LiveCD (Ubuntu, BackTrack, ...)
I recommend using Ubuntu because it is the easiest to create.
There is a little tutorial on Ubuntu's website: http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-mac-osx
(If you Have trouble be sure to ask a question on AskUbuntu)
After you have a recovery usb wipe the disk using your favorite tool. I found a nice tutorial on 'How to geek', but it's a bit outdated. (Tip: download wipe from the software center, not from synaptic)
If your MacBook is capable of upgrading to 10.7 or newer (the current 10.9, OS X Mavericks is available for free from the AppStore, 10.7 - Lion and 10.8 - Mountain Lion are for sale on DVD at the apple online store for under €20,-) you can use the disk utility from the recovery partition (after installing a newer version of OS X, in this case from a bootable USB Drive) by holding cmd + r during boot.
Else, if you are not afraid to open the MacBook you can also remove the drive, put it in an external enclosure and format it with another machine.
I suggest you borrow or purchase an inexpensive external DVD drive to see if you can boot from the install disc. This Sanyo drive is only $28US and works just fine read/write on all the Macs I've attached it to, old and new. I can't guarantee you will be able to boot from it, as I've never tried that, but if it won't boot your laptop you can always send it back for a refund.