This is a bug being forwarded up from Ubuntu:

"The "Xfce 4 Settings Manager, Session and Startup, Application Autostart" can't see or edit properties of existing entry.
IHMO this is somewhat unreasonable and difficult to use. Obviously, you can go and look at the config files (in .config or whereever), but is should be available from the GUI."

https://bugs.launchpad.net/ubuntu/+source/xfce4-session/+bug/405532