Presuming you already have these settings:

Admin > Configuration > Layout
Column Left Status - Global 1
Column Right Status - Global 0


Admin > Tools > Layout Boxes Controller
- everything OFF except for Categories Sidebox


open - includes/templates/pgsnow/common/tpl_main_page.php

near the page top, around line #42 you will see this line:

if (in_array($current_page_base,explode(",",'list_pages_to_skip_all_right_sideboxes _on_here,separated_by_commas,and_no_spaces')) ) {
$flag_disable_right = true;
}

change it to read:

// if you want your Category Sidebox to show on your Shopping Cart page, remove the shopping_cart, part from the line below.
if (in_array($current_page_base,explode(",",'shopping_cart,checkout_shipping,checko ut_payment,checkout_confirmation,checkout_success')) ) {
$flag_disable_left = true;
}

if ($this_is_home_page) {
$flag_disable_left = true;
}

You should also correct your HTML Validation errors as you have many and some are serious.

http://validator.w3.org/ << a site you may want to Bookmark