Also, if you haven't already, try using an absolute pathname. I don't think sh uses /etc/profile (and the load-plugin is basically using sh), so it might not be setting a PATH. I know I've had trouble trying to get it to recognize my ~/bin/ directory.

And make sure you (not just root) have execute permissions on the file.

Also, the "sudo su -" part might be screwing you up - that will start a new (interactive) shell, which will not get any input since the load-applet is running it.

I personally would add your command to the /etc/sudoers file and use sudo to run the command, instead of using "sudo su -".