Instructions
Hold down the "Left Trigger" button
Press "Up" on the directional pad to go up one level. This will focus the camera on that particular level and allow you to interact with items and Sims on that level only.
Press "Up" again with the "Left Trigger" held down to go up any additional levels.
Press "Down" with the "Left Trigger" held to go back downstairs.
Click on a portion of the floor upstairs using the left thumb stick and pressing the "A" button. Select "Go Here" and press the "A" button again. This will direct your Sim to walk upstairs.