I simply followed the install instructions and copied the bit of code into the index.php file. It was one simple step. I don't really know what to check.
Here is the code I copied from the instructions and replaced the bit of code around line 191:
//Master Products
$listing_sql = "select " . $select_column_list . " p.products_id, p.manufacturers_id, p.products_price, p.products_master, p.products_master_status, p.products_tax_class_id, IF(s.status, s.specials_new_products_price, NULL) as specials_new_products_price, IF(s.status, s.specials_new_products_price, p.products_price) as final_price from " . TABLE_PRODUCTS_DESCRIPTION . " pd, " . TABLE_PRODUCTS . " p left join " . TABLE_MANUFACTURERS . " m on p.manufacturers_id = m.manufacturers_id left join " . TABLE_SPECIALS . " s on p.products_id = s.products_id, " . TABLE_PRODUCTS_TO_CATEGORIES . " p2c where p.products_status = '1' and p.products_master = '0' and p.products_listing_status = '1' and p.products_id = p2c.products_id and pd.products_id = p2c.products_id and pd.language_id = '" . (int)$languages_id . "' and p2c.categories_id = '" . (int)$current_category_id . "'";
// Master Products EOF