Playing as a Ghost in The Sims 3 Wii Version: Not Possible

You cannot play as a ghost in The Sims 3 on the Wii. While the PC, Mac, PlayStation 3, and Xbox 360 versions allow deceased Sims to become playable ghosts—with unique interactions and the ability to be resurrected—the Wii version is a streamlined port that omits this feature entirely.

Wii Version Limitations

The Wii release focuses on core life simulation with simplified controls and lacks the expansion content and supernatural mechanics found in the full PC release. If you want the ghost experience, you'll need to play on PC or the HD consoles.