Thanks for raising this here!
Currently, as the feature is still in Beta, inSided has to activate the mega menu for you on your community on request. We have plans to change this very soon as this feature is reaching maturity and adoption levels which give us confidence everything is working as expected.
One thing that we still need to improve is that the mega menu, once activated, is being visible in the new Ideation module as well as in the new Product Updates module. As these are more seperate modules, the mega menu was not working there by default. Improvements like these will most likely come very soon as we want to finalize everything and fully release those modules.
I can manually activate the mega menu in these modules for you already, then you should be able to use this menu also within Ideation!