Main Menu Editor adds new menu but it does not show in LXPanel 0.7.2. Even after reboot. What am I missing?
This is a known issue with unfortunately only a workaround hack thus far that can be found here. It appears as if the problem starts when you hit "Cancel." Of course, "just don't do that" is not a good remedy, but I am afraid it is the only one available right now.
I am not sure I understand your question correctly, but it looks like you edited something in the Main Menu Editor and are expecting items to show up on the Panel(?)
If it is this, you have to separately add items to the panel for them to show up there. Right click on the panel and go to Panel Settings. In the Advanced tab, you can add and remove items.