How do I hook up a washing machine drain line to this box?
It should be a knockout, meaning that a hard sharp blow will pop it out. The plumber should have done this before he glued the drain line in place, since knocking it out now could cause the plug to fall down the drain. This could lead to clogs in the future, since the trap is glued and cannot be disassembled.

Put the tip of a flathead screw driver on the knockout, and tap the back of the screwdriver with your hand or a hammer. Try to get it so that only part of the knockout breaks free. Then grab the loosened disk with a pair of pliers, and twist it free. Be very careful not to drop it down the drain.

Once the hole is open, simply hang the washing machines waste line in the hole. The washer should come with a piece of plastic used to make a hook in the waste line. Install that, and hang the waste line in the hole.