I'm using Gaussian and opendx through ssh channels in X11. In the leopard version of X11, the programs stop to work after a few minutes or as soon as a window is closed.
The installation of X11 Tiger work without problem if we ignore the error stated in my first post (this have been tested on a lot of computer).
This is interesting. I did notice that when I try to use Staroffice in OSX10.5.1, X does not launch. Although I'm not too worried about it. Sun isn't supporting it in the future opting instead for a Java based interface.
The problem for the command you gave in this post is that org.x.X11.plist is not in Leopard's LaunchAgents, or even LaunchDaemons directories.
To see what launchd items are loaded:
sudo launchctl list
You'll probably see only one X11 related item, that being "org.x.privileged_startx".
Sadly, the instructions you are referencing has no mention of the OS the installation id to be applied to (even if you navigate forward or back by thread).
Try updating your system software to the latest (you might have to restore some service configs, but Apple's pretty nice about retaining a backup of your config files for reference. You'll probably need to look for another set of instructions, too...