Are you running this in the Administrator account on that machine? Try that if not. Also check permissions and ownership settings on the Planeshift directory and it's contents (Group ownership on the directory and files is Everyone and Read/Write). If all fails then login as Administrator and reinstall Planeshift from the torrent and then make sure that the Ownership and Permissions is also set as I described above. If this still does not work then I suspect a problem with monitor depth resolution, try different ones. Please check Apple for instructions on these if you need further clarification. I have found Planeshift to be very clean on 10.4.7 and have had no problems...
