So everyone knows that Luke Skywalker is not appearing in the trailer or on the poster for The Force Awakens. But he's in the movie. So there must be a twist to his character. Fans are speculating that he's now Dark Side, but I doubt it. Not off the bat. To figure out Luke's fate, lets examine some clues from the other principal characters that lead to Luke Skywalker.
Rey and Finn are the new protagonists in this new story. They seem to have the same relationship that Luke and Leia had, but they are obviously not brother and sister because that would be predictable and they are cast from two different ethnicities. Poe Dameron is the 3rd protagonist, in the role that Han played in the original series. This leads me to believe that, as Luke, Han and Leia had a love triangle, that Finn, Poe and Rey will have their own love triangle. The twist being that Finn and Rey can end up together because they are not related.
Han and Leia are returning and, although they won, they don't look much happier or advanced since the last time we saw them during the end of Return of the Jedi. Leia is shown wearing very modest royal garments and Han is dressed the same. Han curiously remarks to Chewbacca that they are "home", as if they have been away from someplace, either the Millennium Falcon or Tattoine, where we first met Han and Chewie.
I'm supposing that not much progress has taken place in the 3 decades since The Alliance won. Han and Leia may have been apart in this time or remained together. Either way, I don't see them having a child and abandoning it on another planet, which is where we're supposed to meet Rey. The Star Wars saga is about the Skywalker family, so its most probable that Rey is a Skywalker but not by Leia. Rey does not appear royal like Leia. She appears poor as Luke and Anakin were. Perhaps Luke had her without Leia and Han ever knowing. If Luke is Rey's father, where is he now? This is probably the answer to Rey's quest.
Kylo Ren is the young antagonist of Force Awakens. Not much is known about him, but he obviously has dark Jedi powers. This leads me to believe he is also Luke's child. And like Luke, he will want to fight his father. Only this time, the father will be good and the son will be bad. Will they square off in this film? Possibly. In Phantom Menace, Darth Maul faced Qui-Gon Jinn and killed him. In A New Hope, Darth Vader faced Obi-Wan Kenobi and killed him. These deaths help prepare the young Skywalkers to advance into heroes themselves. Perhaps Luke will fall to Kylo in the same way.
This is what Mark Hamill looked like during the filming of F.A.. The beard and long hair is very reminiscent of Obi-Wan and Qui-Gon, furthering my theory that he is now a Jedi master living in solitude, perhaps to protect his loved ones from becoming targets. Perhaps he has gone even further and become like Yoda, so secluded and powerful, that only the strongest Jedi can find him and must if they hope to become great Jedi. Remember that Luke learned under Yoda so he may have similar methods. If so, Luke may not play a pivotal role in the new trilogy under later, when Rey and Finn are strong enough to encounter him. He may show up at the very end of Force Unleashed.
Something to factor in is how misleading the Star Wars series can be. Darth Maul was thought to be the next major villain in the 2nd trilogy, but he was killed in his one appearance, being only a "Phantom Menace". Kylo Ren may not play a huge role in the new series, only filling in as temporary villain like Maul. Filling in for whom? Luke. If Kylo Ren is Luke's son and he is Dark Side, the only option for Luke is to kill his own son. If Luke does this, he may succumb to the Dark Side himself and become the main villain of Episodes 7,8 and 9. Luke would finally become like his father Anakin. The Emperor said that it was Luke's destiny to become Dark Side. Perhaps Luke will become what The Emperor was and take an apprentice in either Finn, Rey or Poe. Or perhaps he will not kill Kylo but become Emperor by his own will.
So my theories lead me to believe that Luke will appear late in Episode 7, possibly dying or surviving to become Dark Side. So to answer the question, "Where is Luke?": Luke Skywalker is hiding from both the Alliance and the Empire. And he can turn either way. Its a question of "Who finds Luke first?" Kylo or Rey?