I believe this works with windows, but I've never used an iPod on Windows.
Plug your iPod into the Windows computer as normally. While it is plugged in, in iTunes, go to preferences, and then the iPod pane. Then choose the general Tab within the iPod pane. The last option should be "enable disk use." Check this option.
You should now see the iPod as a removable disk in My Computer. Open it and copy your files to it (anywhere except the folders which already exist -- make a new folder or put them on the root of the iPod).
Now, eject the iPod in iTunes (very important -- DO NOT unplug the iPod when disk use is enabled unless you've done this).
Plug it in to the Mac. Again, in iTunes, go to preferences and then the iPod pane, and check the same box. Your iPod should now be on your desktop and you can open it and copy over the contents.