First you need to beat the elite four and have the Natinol pokedex.

THen go to stark Mountion.

Then you go inside STark MOuntion and go to end.

One the way you will meat a person named Bark.

He will be your Partner until you get to the end.

When you reach the end Bark will take the item that is there and run of.
THEN GO BACK to SUrvival area and in one of the Houses is Bark.
Talk to him and he will go back to Stark Mountion.

WHen you go back to the place you saw the item.
Heatran will be there.

Hope i Helped you.