It depends, if you bought the Stag 15 DMR or the VN model. The DMR has a detachable carry handle which covers the upper reciever rail, but with the VN model the carry handle is part of the the upper.
DMR--- just take the carry handle off and then you can attach your optics directly to the upper
VN--- you will need to buy a carry handle optic mount which is carried by TA
http://torontoairsoft.com/product.php?id_product=209