The main menu for Casual RE-Builder is casual-re-builder-tmenu. Bind this menu to your preference in the keymaps reb-mode-map and reb-lisp-mode-map as follows in your initialization file.
(keymap-set reb-mode-map "C-o" #'casual-re-builder-tmenu) (keymap-set reb-lisp-mode-map "C-o" #'casual-re-builder-tmenu)