After getting the national Dex go to stark mountain then the first time you go team galactic should be there then after they arrest the old guy go to the trainer cafe then talk to buck after that go back to stark mountain and Heatran will be in the same place where they arrested the old guy.
You must have beaten the elite 4 Pokemon league, then check your Sinnoh Pokedex and see if it is full.
1.If it is, fly to Professor Rowan's lab and talk to him.
2.In pokemon platinum there should be 210 Pokemon.
3.When you obtain the National Dex, go to Snowpoint City and talk to the guy next to the boat.
4.You'll arrive at the battle grounds and then battle a load of people .
5.When the routes to the Resort Area and other areas are unblocked, head to the mountain.
6.Then beat team galaxy, go through the mountain, go out of it when you've go to the last bit, and finally go out again.
7.Then talk to buck.
8.Go inside again and you'll meat Heatran!