Even if you get it working, I'm experiencing too many owners losing it all. Here's the story and why.
1. To share a volume between Mac and PC the current common file system is FAT32.
2. FAT32 is fragile. Don't write to me about this or ask how to fix it.
3. If I have the FAT32 USB/FIREWIRE connected drive and I pull the USB or FIREWIRE cable out without ejecting the drive or doing a "nice" shutdown, the drive contents can go missing. Again, don't write to me asking how to fix this.
4. OK, that should be enough information and warnings for you to get a FAT32 partition formatted and exchanging files with the PC and MAC.
What I want to stress here is such a volume is only good for exchanges and never to be trusted as your only copy.