Go in front of one of the houses in Nuketown (it doesn't matter which one) and go to your right.

Before you get in the backyard stop at the corner of the house and face what looks like a flower box. It should say Press A to get on top.

After you do that climb up the wooden lattice that looks kinda like a ladder. You shold be in front of one of the windows on the top floor.

If the window is already broken you can climb inside.