It looks like you have to do it in arcade or 360 mode which means you're going to have to learn to score. A task not to be undertaken lightly in this game. The first extend is at 15 million and the second at 35. There are at least a couple of videos on youtube of people getting that many points http://www.youtube.com/watch?v=m5_jPOaLGFM
I tend to play this just play for survival, I would get the second extend on stage 3. My best is getting to stage 5 with about 50 million and I was well chuffed to beat the stage 4 boss.