Here's how to get Spiritomb on Platinum.
First, go to twinleaf town. Make sure you have a Pokemon with surf. Then go to the lake and use surf. Go to the borders of the lake and click A. Soon you'll have a found a odd key stone. Take the stone town to solaceon town and go south. go to the shore and you'll see a rock. Click on the rock and say yes when it asked if the odd keystone should be insterted. The tower will be ressurcted and your half way done.
Get a friend who has a nintendo ds and Pokemon platinum. Go the underground and use your digger drill to make a secret base. there's a trick that makes it so that you can meet the same poerson 32 times. Talk to your friend in the underground and enter the secrtet base. Go out of hte secrtet base and talk to your friend again. Repeat this 32 times and the game will be tricked into thinking you've talked to 32 people.
Go back to the tower. click on the ressurcted tower and spirtomb should appear. Watch out- its very hard to catch. Good Luck and hope this wasn't confusing!
Source - experience
Hope I Helped!