How do you connect a Macbook to a TV for a keynote presentation?

http://store.apple.com/us/browse/home/shop_mac/mac_accessories/displays?mco=Mzk2ODE3OA

or

http://store.apple.com/us/browse/home/shop_mac/mac_accessories/cables?mco=NTMxNDY

should have the answer to your question.

What would be more helpful, when the macbook was purchased?

yeah, you need a mini dvi to VGA adapter, or if you’d rather support a high resolution and it look sharper, you can go Mini DVI – DVI and adapt it to HDMI. Very easy to do, and you’ll need one sound cable as well, assuming you plan to play any audio for your keynote presentation.

If you have a new MacBook (aluminum with the black border around the screen) it uses a Mini DisplayPort. So you would need this to go to DVI. The odds are pretty good that your TV has a DVI input.

actually DVI is a bit uncommon from what I’ve seen. But you can adapt DVI to HDMI.