After getting into the dream they also have that dream sharing machine with them. How did they get that machine 'working' (fully functional) inside dream because inside dream that machine was just an image of the real machine.
It's all part of the consciousness being shared by the dreamers. Basically in a dream world you can create whatever you want. Therefore, inside the many levels of dreams, they can create the dream machine being there, along with just about anything they want. Edit: Because the dreamers perceive that everything occurring is real, the dream machine is then perceived to be functional causing the next level of dreaming.
In fact, this is a big impossibility, not just a "litle thing with no importance". The argument of "controlling the dream towards a second and a third level" would be aceptable if a one person dream. We are talking about 7 people that share these diferent levels--too much of a distortion. The natural way to go would be each one of them walk their unique way since the second dream level. I can't accept this size of divergence.
Having said that, I close my eyes to that because I loved the movie. What I loved about the movie was the exploration of the mind features witch I think to be accurate.