I have a Netgammon game application which I have been playing for about two years. Today, it will not allow me to play the game with someone on-line. I can only play a solo game against the computer. A message pops up asking me to enter my Apple ID. I hesitate to do this because I don't know if this message is from Apple, or the maker of the application. Apparentlly they are asking me to sign in to the "Game Center" based on the title of the message. Can someone help me out on this?