Where is the tilde on French and US keyboard layouts? I have configured my keyboard for French and US layouts. I cannot find a tilde anywhere on there. 

Normally, the tilde (~) is on the upper case of the key to the left of the "1" character.

It is pretty essential if you use the terminal a lot, as it allows you to go to directly somebody else's home directory. cd ~suzanne takes you to suzanne's home. Since, as a Linux user, I make a lot of use of the terminal, I really need the tilde.
Choose the keyboard viewer from the input menu on the menu bar. If you have the visible Keyboard in front of you, you can find out which keyboard combination for the Tilde is in both US and French 

See also Apple Support Page to enable this feature.