Use the navigation stick to walk up to the treasure chest.
"Fable 2" is a video game for the Xbox 360 platform that allows you to fully explore a role-playing environment as a character other than yourself. Fable 2 features a comprehensive loot system that involves both drops from defeated foes and treasure chests that appear throughout the game. Each chest can be opened with the appropriate key combination.
Wait for the dialog saying "Press the A Button to Open the Chest" to appear.
Press the "A" button on your Xbox 360 controller to open the chest.