[Plugin: Woocommerce] getting quantity of a product from a completed order

I have been working on a credits system for WordPress, which I installed the plugin WordPress Types. I setup my usermeta field and suddenly found myself stuck on how to add the current value of the credits onto the quantity from the latest completed order. I know where I need to put my code, but I have no idea how to extract the quantity from the order. I’ve spent all day looking through the Woocommerce API and don’t understand how to get the information that I need from it.

<?php
function add_credits($order_quantity, $order_id){
	global $wpdb, $current_user, $order_quantity;
	get_currentuserinfo();
	get_order();
	$current_credits = types_render_usermeta_field("credits", array("user_current" => "true", FIELD_VALUE));
		if ($current_credits = "NULL"){ $current_credits = 0; }	// may not have bought credits yet
	$newcredits = $current_credits + $order_quantity;

	$wpdb->update("$wpdb->usermeta", array('wpcf-credits' => $newcredits), array($currentuser->id));
}
?>

That’s the code that I have for now, but I know that I am way off and I was wondering if anyone could set me on the right course?

Thanks!