I'm not familar with HFS/HFS+ (OSX filesystem) support under Linux, but it may be the case that the Linux LiveCD is just respecting the OSX permissions by not giving the LiveCD user access to your OSX user's files.
What does "mount" in a terminal report about the HFS volume? What does "ls -l osx_mount_point" report?
And you can use root/superuser to override. "sudo -s" in a terminal will start a root shell. Or you can try "Alt+F2" to start a run dialog, then "gksudo nautilus" to start a root file browser (I don't know how well this works).