I have the same problem. Sorry to say I have yet to find a fix. I've tried multiple fire wires. It worked fine on my old Mac Book Pro running Leopard and iMovie '09, but will not work on new Macbook running Mountain Lion and iMovie "11. It just won't recognize the camera. It almost seems like an imcompatibility with Mountain Lion.
Tried multiple alternate import softwares such as Roxio. Tried removing all other media import software such as Canon and Nikon transfer. Only thing I haven't done is remove and reinstall iMovie.
If you find a fix let me know. Very frustrating.