Engine block drain plug. Should be on the passenger side, on the engine block.

To do a good flush, you need to first drain the radiator, then open up the block drain plug, and close the radiator drain. Set climate controls to HOT. Get a garden hose and stick it in the radiator neck. Flush water down the radiator and it should start pouring out of the block. Keep flushing until you get good clear water coming out.

After the block is done draining, open the radiator drain back up. Let everything drain out. Reinstall block drain plug and shut off radiator drain.

Either put the front end up on jackstands, or park on a hill with the nose up higher than the rear. Slowly fill the radiator up until its full. It may burp a few times. Leave a LITTLE bit of room in the radiator, dont overflow it. Start the truck up. LEAVE THE RADIATOR CAP OFF. Stand there next to the radiator with the cap in hand, ready to reinstall the cap if it starts to overflow. If you have any air pockets, it will bubble up out of the radiator. Let the truck get up to normal operating temp (gauge a little below halfway) Top off the coolant if it gets low. Look for coolant flowing through the radiator once the thermostat opens up. After this happens, you can install the radiator cap, and you should be good to go. If the thermostat never opens, try replacing it. You could still have some air though.