Here's how it works:
1. Find the Old Key Stone: It's located in the Stark Mountain area, deep inside the mountain. You'll need to use the Strength HM to move boulders and reach it.
2. Find Heatran: Once you have the Old Key Stone, you can find Heatran in the Stark Mountain area as well. It's located in a special room within the mountain.
3. Use the Old Key Stone: To awaken Heatran, you'll need to use the Old Key Stone on it. This will trigger a battle with Heatran, allowing you to catch it.
Important Note: Heatran can be very powerful, so make sure you are well prepared before challenging it!