In Pokemon Platinum, fossils are revived into Pokemon using the Fossil Reviver at the Oreburgh Mine.
Here's how it works:
1. Find Fossils: You can find fossils in various locations around the game, often by digging with your Dowsing Machine.
2. Take them to Oreburgh Mine: Go to the Oreburgh Mine and talk to the scientist inside.
3. Use the Fossil Reviver: The scientist will use the Fossil Reviver to bring your fossils back to life as Pokemon.
Note: Different fossils revive into different Pokemon. For example, the "Helix Fossil" revives into Cranidos, while the "Dome Fossil" revives into Shieldon.