When you plug a new one in, you have the option to run through a "layout detection" process. Typically it asks you to press the keys directly adjacent to the "s

When you plug a new one in, you have the option to run through a "layout detection" process. Typically it asks you to press the keys directly adjacent to the "shift" keys and goes from there. Also, IME, there is not a "mac" keyboard on a hardware level - the key directly to the left of the space is alt on Windows or Command on OSX.