Customizing the Openbox Menu
There are two ways you can make changes to the openbox menu. You can edit the menu.xml file, which is usually located with the other openbox configuration files (in ~/.config/openbox/ on Ubuntu 12.04).
There's an easier, GUI-based way -- using a program called obmenu. Once you have obmenu installed (sudo apt-get install obmenu), simply type
obmenu in a terminal. A window like the one above will come up and allow you to change the menu that pops up with a right click on the openbox desktop.
You can find some pretty handy openbox menu.xml files on the Internet, also. I personally like the one that comes with a GNU/Linux distribution called crunchbang: Default #!crunchbang openbox menu.xml