Bryan Singer Directing X-Men: Days of Future Past

[Mulholland '49]

Originally Posted by The Guard

It's really not rocket science.

Xavier is talking about the fact that Magneto helped him build the particular big round Cerebro room that they're standing in during X-MEN when he says "Because he helped me build it". Ergo, Magneto helped Xavier build that particular room, which, in X-MEN and X2, is the only Cerebro anyone knew had existed at that point.
.

No...

WOLVERINE: So why can't you use it (Cerebro) to find Magneto?
CHARLES: I've been trying, but he's found some way to shield himself from it. (Cerebro)
WOLVERINE: How would he know how to do that?
CHARLES: Because he helped me build it. (Cerebro)