If Dalai Lama is enlightened, why he keeps coming back? Assuming the result of enlightenment is attainment of Nirvana, and assuming Dalai Lama is enlightened, why does he keep on getting reborn?

Could it mean that Nirvana means something else, for example a state of egolessness while still a person on planet Earth?
This is a fundamental premise of Mayayana, that the goal is not to be extinguished and leave samsara like an Arhat, but to stick around until everyone is enlightened. 

Here is the zen formulation of the Bodhisattva Vow (couldn't find the Tibetan version)


  Beings are numberless; I vow to save them. Delusions are
  inexhaustible; I vow to end them. Dharma gates are boundless; I vow to
  enter them. Buddha's Way is unsurpassable; I vow to become it.


As for where one is upon enlightenment, there are more lots of formulations, I don't have a good idea on how to summarize them.