How can I use Mighty Mouse Swipe Gestures for Back/Forth in Mac App Store?
Unfortunately, the native Magic Mouse preferences pane is a little sparse.  As far as I can tell, unless you are willing to move to a 3rd party solution, the Magic Mouse does not support the behavior you are looking for.  In order to enable this functionality on an Apple trackpad, one must change the preference from 2 finger swipe to 3 finger swipe ("2 or 3" finger works as well).  It should be noted though, that even when enabled, in apps without the fancy page sliding across the screen animation, the swiping does not follow the "Natural Scrolling" paradigm, as in you must swipe left to go back and right to go forwarded.  

While not the answer you were hoping for, it sounds like your best bet would be to either investigate a 3rd party option like MagicPrefs or even better, invest in a Magic Trackpad.  The gesture options, even natively are greatly expanded.  It will open up a whole new world of gesture based computing for you.