There are two "alt" keys on a mac keyboard. For a Swiss keyboard the the "@" sign is obtained by pressing "alt" and "G" at the same time assuming you've set the input source as a swiss keyboard.

However, there's a catch. If you're running another system other than Mac OS, native or virtualised, you may find that the left "alt" key will no longer work in conjunction with the "G" to produce the "@" symbol. You need to use the "alt" key on the right side of the keyboard for this to work.