How DO I Set Up The Rodent Pen?

To set up the rodent pen, or any animal's habitat, go to the room of the house that has the computer in it and press the button on the bottom of the touch screen that looks like the character's face. Then touch the "Expansion" option, and select the type of pet pen you want to buy or upgrade. Then you enter the room next to the computer room that has nothing in it( the room that is small and to the lower right of the computer room. Last click on your face icon (the one at the bottom of the touch screen)while you are near the stairs and it should bring up a menu referred to as "Living space". Next click "Finish day" the next day you should start to get customers, rabbits and other animals in other scenarios.

It wont let me buy nor will it let me finish day

I was able to get it only in the upper left corner of the room mentioned.. So go into room below the computer room and then to the upper left of that room, near the stairs. Then tapping on the face or 'A' key will give that menu...