Someone gave me the Snow Leopard upgrade for Christmas. But my IMac (Intel) has Tiger 10.4.11. My understanding is that you have to upgrade to Leopard first. So I bought the 10.5 software. But my computer won't read the disc. It just spits it out with nothing showing on the screen. The version I bought says Max OSX Leopard Install DVD 2Z691-6037-A. What am I doing wrong?