Key to boot from USB on Ubuntu Macbook 2007
First step is to discover if the OS on the USB stick is properly formatted and built for a boot device.

For this you will need a Mac running OS X. Insert the stick and go to System Preferences → Startup Disk. If your stick is ready to boot a Mac it will appear in the selection pane :-



If it is there then your USB stick is bootable. Just insert it into your MacBook and hold down the Option key when it starts up and it will let you choose the boot device. Use the arrow keys to select your stick and then hit return.

If it doesn't show up the most likely reason is a badly formatted stick. Your stick must use a GUID partition table and the boot partition must be formatted Mac OS X Extended (Journalled).