Dude, it's not included in the box. Buy it at Radio Shack or something. Or buy the Apple Travel kit for a lot more money.
Don't take the extension cable unless you know you need it. On the brick/charger, it's two-prong. So plug in a simple two-prong adapter. For example, 110V two-prong in US is two parallel flat metals. 220~240V Paris cable was two round metals.
European countries all have different types, so if you're traveling just to Amsterdam, buy an adapter for the specific country. If you don't know, just get a travel kit.