typing a backtick on Mac OS X Snow Leopard
Disclaimer: This info is for English keyboards. If you're using a keyboard for a different language, look for the keys on your keyboard that match the photos.  Pictures of Apple's keyboards for various languages can be found here.

This question was answered 5 years ago, but pictures may be helpful for viewers coming upon it now.

To make a backtick on the US English keyboard - Use the key just to the left of the 1 key



To make a backtick on the British or International English keyboard - Use the key just to the right of the left shift key