Isn't the Earth's magnetic field too weak to lift anything?

Indeed, the Earth's magnetic field averages a mere 0.5 gauss at sea level. That's about 10,000 times weaker than the field around a small permanent magnet. Yet what the geomagnetic field lacks in flux density, it more than makes up for in size. In fact, one cubic kilometer near the surface packs a magnetic energy density of about 1.4 million Joules! Theoretically, a craft could be levitated by the geomagnetic field in much the same way a helium baloon rises. A balloon rises because it displaces more weight in air than its own weight. Similarly, if a magnetic field displaced the energy equivalent of the geomagnetic field needed to overcome gravity, then it would float as well.