Unfortunately, since it's not bvh or fbx format, you will probably have to convert it. I haven't had time to try it yet, but Motionbuilder is supposed to be able to import *.c3d files. Anyone who has experience with handling this kind of data, please feel free to chime in.
Even if you can't use the MOCAP data directly, they also have video and animations that you can use as references for the motions.
Mods, what about a MOCAP resources sticky?