How to Make a Ghost Come Out on "Sims 3"by Seth Amery ; Updated September 22, 2017
If you're interested in adding a new ghostly roommate to your household in The Sims 3, you have a few options at your disposal. The most common way to get ghosts on your lot is to have a Sim die in your house, although you won't have control over it right away -- unless you resurrect the ghost at the science lab. On the other hand, if you have the Supernatural expansion pack, you can create ghosts in Create-a-Sim to play like a normal Sim.
Pick up the urn after a Sim dies and place it into your Sim's inventory. He'll need to bring the urn to another lot anyway, and this protects the urn from destruction should something happen to the house.
Accept the Oh My Ghost! opportunity when it appears. This special event occurs a few in-game days after a Sim dies on your lot, and only if your Sim was close to the victim. It only demands one task: Bring the urn to the science lab in your town.
If you don't know where the science lab is, you can just click the star icon that appears after accepting the opportunity. The star icon always appears on your screen relative to the direction of the opportunity building; if the science lab is north of where you are, the icon appears near the top and points up.
Head to the science lab. After a few in-game hours, the no-cost opportunity will complete and you'll have a new ghost that acts like any other homeless NPC in your town; that means you can become friends and ask the ghost to move into your home. If the resurrected Sim was already part of your household, she will remain so; however, any previous romantic relationships need to be restarted and rekindled.
If a stranger dies on your lot, you can keep the urn on your lot to allow his ghost to visit at random times, or you can take the urn to the graveyard and place the ghost to rest.