In The Last Stand Magneto didn't have his powers after Beast injected him with the cure. But in Days of Future Past he had his powers. If Days of Future Past was part of the original X-Men series, isn't Magneto supposed to be dead in the future since he is a human or at least not have his powers?
The epilogue ending of X-Men: The Last Stand depicts Erik playing chess and realizing that he can move the piece just slightly with his powers. The cure is clearly not as final or effective as everyone had been led to believe, and his powers had begun to return.