In your index.php file, check that the SMFShop section looks like:
// Begin SMFShop code
'shop' => array('shop/Shop.php', 'Shop'),
'shop_general' => array('shop/ShopAdmin.php', 'ShopGeneral'),
'shop_inventory' => array('shop/ShopAdmin.php', 'ShopInventory'),
'shop_items_add' => array('shop/ShopAdmin.php', 'ShopItemsAdd'),
'shop_items_edit' => array('shop/ShopAdmin.php', 'ShopItemsEdit'),
'shop_restock' => array('shop/ShopAdmin.php', 'ShopRestock'),
'shop_usergroup' => array('shop/ShopAdmin.php', 'ShopUserGroup'),
'shop_cat' => array('shop/ShopAdmin.php', 'ShopCategories'),
// End SMFShop code
Make sure that "shop_cat" is there.