First, make sure the tablet is connected (Wacom provides a somewhat generic driver that works with multiple different devices so it won't be able to know what choices to offer you until it see which tablet is connected to the computer.)
Second, open "System Preferences" and select the "Wacom Tablet" preferences.
Third, near the bottom of the Wacom tablet preferences, you'll see an "About" button and "Options..." button... click the "Options..." button.
Lastly, on that panel there's a section titled "Handedness" with choices "Right" and "Left" (Right is the default). Just change it to "Left" click "Ok" and exit out of all those panels.
I would guess there's a similar looking panel on Windows, but as I don't have Windows I cannot confirm that.