To find out the name of the menu file that you're using, type:
grep -i session.menuFile ~/.blackboxrc
That will return the path and filename that Blackbox is using. Open that file in an editor and read the syntax help at the top of the file (you should be able to figure out how to add an entry just by looking at what is already there).
If you want to use a GUI program to edit the menu, try bbconf
. This tool can also edit styles and bind keystrokes to commands (with bbkeys