You can book seats (at least on an internal flight, assume it holds for an international one) using the LOT manage my booking facility (I'm assuming your booking was made on the amedeus ticketing system);
http://www.lot.com/gb/en/web/newlot/...ur-reservation

Should allow seat selection - note that many of the rows may appear blocked, it appears as though Air China has an odd numbering system for flights so the plane economy section may start from row 10 or even higher