"chmod 777 /Mountpoint" where /Mountpoint is where it's mounted. If you already have a directory structure there the -R option will apply it to all levels.
However, if a root file manager didn't work you need to check a couple of other points:
1. Make sure the drive gets mounted read-write. No amount of permission will enable you to write to a read-only file system.
2. Make sure msec (PCC security section) isn't set up to change the permissions back to default every hour. If it is you'll have change the permissions there as well.