Whether you're playing Assassin's Creed: Brotherhood on the PlayStation 3 or the Xbox 360, finding the Shrine in Basilica di San Pietro (St. Peter's Basilica) is the same. And when you do find it, you'll earn yourself the "Amen" trophy or achievement. Just follow along with this video and see exactly where the Basilica di San Pietro Shrine is. Finding the Shrine is well worth it; Silver or 20G. Amen to that.
This video specifically covers the gameplay on the PS3, but it should be the same across the board. Once you find all 6 Shrines in Brotherhood, you can unlock the treasure in the secret Lair of Romulus.