It was a great game!

I have been playing this game for several years and I really enjoyed this game until recently. When I try to play this game now, I am being asked to sign in to Game Center. When I go to the App Store and key in Game Center, it gives me a list of game, but, no Game Center. How can I play if I cannot even get the Game Center?

Mr JRC about MetalStorm: Online, v5.2.1