Instructions
Press the "pg up" button on your keyboard. This will send the camera's focus to the second floor of the home.
Click your mouse in the area of the second floor you would like your Sim to stand. You can also click on an item for the Sim to interact with on the second floor.
Click on the Sim's portrait on the bottom of the screen. This will shift the focus of the game to the Sim you commanded to walk up the stairs.
Right click the mouse to lock the camera. The camera will follow the selected Sim so you can determine if he has followed your command to walk up the stairs.