How do I type an `@` (at sign) symbol?

Make sure you selected the right keyboard layout. Go to System Settings, click on Keyboard Layout. On the Layout tab you should see a + button. Select the most appropriate keyboard layout as shown bellow.

I am not using a Macbook, but the first layout worked as you described to type @. I suppose you are using Ubuntu 11.10.

In Ubuntu 14.04

System Settings -> Keyboard -> Shortcuts -> Typing And define the "Alternative Character Key" I used Right Ctrl Key.

Combine this key with Q or 2.

Result: @

With Ubuntu 12.04 on a Macbook Danish keyboard, I get the @ sign with the right Alt+2. It doesn't work with the left alt key as normally on the Mac.

Ctrl+Shift+U+40 would do the trick!