The Wardrobe

At the party house, talk to Morpheus to learn he has lost his blue pill. Once you have coins, enter the phone booth outside the party house and dial 0100 / 1110 / 0110 / 0101 / 1011 / 11 / OK. Pick up the blue pill from the coin return slot (you will only get it if you've talked to Morpheus) and give it to Morpheus.

You will end up in the developers room so play around if you want but to leave, knock on the door and ask how you can leave.