Have a saved game file from Sonic The Hedgehog 4: Episode 1, then complete Sylvania Castle Zone Act 1. Go to the "Map" screen and you can now play as Metal Sonic in Episode Metal.
Super Sonic as a Playable Character
When you complete each of the special stages and obtain ALL 7 Chaos Emeralds Super Sonic will become available as a playable character. Note: You must collect 50 rings in a given stage.