Near the house of Chev (the main character) there is a pipe through which you can go to Chinatown. There will be a chef sitting there, but nothing can be done with him until you go further and meet a little champ and talk to him. After that, take the cheese to the chef, and then when he gives you the food, you need to take it to little champ, and he will allow you to enter the door. After that, you need to talk to a rat that's just standing there (I don't remember the name) and then go back to the pipe, and then go through the brick wall (the door, according to that rat).