Crinno,
Open up System Preferences --> International --> Input Menu
Under "keyboards", you should have relevant option checked, and none others - I see both Swiss French and Swiss German, both bring up the @ symbol for me.
If this fails, in the same preferences screen check "Show Input Menu in Menu bar" - select the flag that appears in the menu bar and choose "Show Keyboard Viewer" - this will bring up a map of your keyboard - see if you can locate it on there.