1. Defeated the Elite Four and become the Champion.
2. Spoken to the President of the Battle Frontier, who is located at the top of Victory Road, just outside the Indigo Plateau.
Once you have spoken to the President, the Battle Frontier will open up and you can challenge the various facilities.
Here's a quick guide to finding the Battle Frontier:
1. Head to Victory Road: This is the path leading to the Indigo Plateau. You can access it from Pallet Town after you've defeated the Elite Four.
2. Go to the top of Victory Road: You'll find the President near the entrance to the Indigo Plateau.
3. Talk to the President: He will inform you about the Battle Frontier and allow you access.
4. Enter the Battle Frontier: You can now enter the Battle Frontier from the entrance in Victory Road.
Note: The Battle Frontier is a post-game location, so you won't be able to access it until after you've completed the main story of Pokemon SoulSilver.