Das Werkstatt

Here's one approach that worked for me, trying to boot a Xubuntu Linux (16.04) install DVD:

1. Shutdown the Macbook
2. Press the power button
3. Hold the "Alt" key (option key) - Holding "C" won't work.
4. When the boot-select screen shows up, it displays the built-in harddisk "Macintosh HD".
5. Insert the Xubuntu DVD
6. Wait...
7. Now, 2 new boot options should appear: "Windows" and "EFI boot"

The "Windows" is confusing, but that's Apple's way of saying "something non-Apple"
I've selected "EFI boot".

Links:
Startup key combinations for Mac (support.apple.com)