I have not tried it on my Windows platform as of yet, but I also have been having problems loading it onto the MacBook Pro, Mac OS X Version 10.5.1, Leopard.
After reading the JVC manual, I discovered that this camcorder DOES work with MAC, but only the MAC OS X Versions 10.3.1 to v10.3.9, v10.4.2 to v10.4.8.... So, I made a bad buyer's mistake and didn't check out the Versions of Mac that the camcorder could adapt to. I have seen other bulletins, though, where others have been able to operate their GZ-MG155 camcorders WITH the version 10.5.1.
Be sure that when you are trying to access the files on the hard drive in the camcorder that you have the camcorder on the platform that comes with it. The platform should be plugged into the wall and should be feeding the camcorder power. Plug the USB connector directly into the laptop- not into an extension USB port, and the other end into the PLATFORM- not into the camcorder itself. Once it is connected in this way, you should turn the power switch to ON on the camcorder and flip open the screen. Once you see the Everio logo on the screen it should bring you to the option of "connect to PC". Choose this option. The Everio hard drive will appear on your desk top.
From here, you open imovie '08. The JVC website has a search box in the upper right hand corner. If you type in the camcorder model a list of applicable articles, FAQs, etc will appear. In this list (search 'site') will appear a link to a tutorial to using the GZ-MG155 with Mac OS X.
I will research that link and post it when I find it again.
Remember this one very important fact: If you have more than 50 movie files or if the total data size exceeds 4300 MB, then your imovie '08 may end up crashing. I don't know if you will end up losing the movies along with it- so be careful with this!
Again, I have yet to get mine to work, but I don't have my platform with me, so I can't try it just yet.
Good luck with this and I hope I've helped!!
I'll reply with that link when I find it again.