The "~" key is located between the "left shift" and "Z" keys on the international english keyboard.
Many users are used to have this key under the escape key (the US layout), and they (including me) use the KeyRemap4MacBook software to swap those keys.
Edit: KeyRemap4MacBook is now called Karabiner. Here is a screenshot of the configuration I'm using to remap Section to Backquote key.
Edit: Karabiner does not work on macOS Sierra, however a new project is in development: Karabiner Elements. As of now, it can do the replacement in question:
