The center of the Mojave is the New Vegas Strip. It can be reached by either Freeside or the monorail in Camp McCarren.
The Strip isn't the physical center of the Mojave, though... spiritually, maybe, but not physically.
The physical center is over to the east of quarry junction. The Strip is the center of society in the Mojave, and I guess you could say the Hoover Dam is the strategic center, since that's what everyone is fighting for.