Essentially you will need to check if the user has scrolled to or beyond the div id of about.
First you will need to establish the current Y value of the div.
//cache about div
var about = $('#about');
//this caches the about div position on window load
var aboutPosition = about.position();
Next you will need to determine how far the the user has scrolled. The best way I have determined to accomplish this is with a timer. You could use the scoll event but its far too taxing on the user browser and a timer will be for the most part indistinguishable.
//generic timer set to repeat every 10 mili(very fast)
//with a callback to execute the logic
var checkScrollPos = window.setInterval("scrollTopLogic()",10);
function scrollTopLogic(){
//if about y position is greater than or equal to the
//current window scroll position do something
if(aboutPosition.y >= $(window).scrollTop()){
$('nav').show();
//remove timer since it is no longer needed
window.clearInterval(checkScrollPos);
}
}