First off, get Sonic Robo Blast 2 here. If you have v1.08, UPDATE. If you've downloaded it since 9/11/05, then you're in the clear.

Let's put it quite simply... Beat the game three times. You don't have to do them all in one sitting, but you do have to ensure you've beaten the game three times already. Chances are, you already HAVE beaten it three times. Once you do that, jump up and down on the house in GFZ1 where Sonic's emblem was (nearer to the beginning than the Gold Shield).

You'll see.