If you open your smoothscroll.js file, at the end you will see this:
// Initialize all .smoothScroll links
jQuery(function($){ $.localScroll({filter:'.smoothScroll'}); });
This means that, when loaded, the library only initializes existing links and does not take into account the rest of the links loaded afterwards with .load().
The solution for this problem is to initialize the loaded a.smoothScroll again. For that you can change your JS code to this:
$(document).ready(function() {
$("#about_div").load("test_scroll.html", function() {
$("#about_div").localScroll({filter:'.smoothScroll'});
});
});