notice that when I right-click on the menu icon, and choose to edit the
menus, there's a complaint to install menulibre or mozo.

however I can already go to the control center and choose the Edit menu
from there. if I install menulibre or mozo, it allows me to edit the
menus directly from the brisk-menu start icon, though in the Control
Center there would now be two items called 'Main Menu'.

I found a workaround,

edit ~/.xsessionrc

add:
PATH=/home/username/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games

go to ~/bin
ln -s `which alacarte` mozo

relogin the graphical session, and the right-click the menu button and
choose 'Edit menus' should now work.

(note: editing ~/.profile to update PATH does not work for this workaround)