JQuery :: Post Returns Data But Does Not Display It

$.post, it calls a php file which retrieves data from my database and should return a JSON datatype result. I checked on firebug, and it does successfully retrieve the data in JSON format. I however cannot get it to display on screen. I want to populate input fields with the result accordingly.

Here's a chunk of my javascript: (this function is being triggered by a dropdown)
function updateBillingAddFields(address_book_id) {
jQuery.post("ajax_checkout_confirmation.php", {
address_book_id: address_book_id,
action: 'get_shipping_add_details'
}, function(data){
var form_element = document.forms['address'];
form_element.elements['entry_name'].value = data.entry_name;
form_element.elements['entry_addr1'].value = data.entry_addr1;
form_element.elements['entry_addr2'].value = data.entry_addr2;
form_element.elements['entry_addr3'].value = data.entry_addr3;
form_element.elements['entry_addr4'].value = data.entry_addr4;
form_element.elements['entry_postcode'].value = data.entry_postcode;
form_element.elements['entry_state_name'].value = data.entry_state_name;
}, "json"); }
I tried putting alert inside my function, and it also doesn't show. I'm thinking it doesn't go inside the function at all.