I only speak Nikon, but there are two settings in those cameras, one for "mass storage" and one labeled something else (in Nikon MTP I think). Basically does the camera pretend to be a disk drive, or pretend to be a camera.
See if you have such a setting, and try the opposite of whatever it is on.
Note that I think LR3 is limited to support for only some cameras, so it still might not work.