Instructions
Wait until the in-game time is after 11 p.m. The clock on the toolbar always displays the Sim time.
Select the Sim who would like to see a ghost from the list of Sims on the left side of the screen.
Scroll the middle mouse button until you zoom out all the way out from the Sim house. You will see a magnifying glass appear.
Click the magnifying glass to view the entire city.
Locate the nearest graveyard. This will display a blue icon with a gravestone on it.
Click on the blue icon and select "Visit" from the options that appear. Your Sim will automatically leave the house and travel to the graveyard.
Right-click on the Sim's portrait to focus on the Sim and see what he is viewing. Your Sim will be able to see and interact with the ghosts in the graveyard at any time between 11 p.m. and 6 p.m. in-game time.