If Beaver Builder is activated it should just be automatic to have the option available. If you exit Microthemer then open it again does the option appear? If that doesn’t work try logging out of the site and clearing your browser cache, then log back in and try again.
@maxp0wr, the BB option should appear in the View menu automatically if BB is installed and activated. So if Abland’s tips don’t resolve the issue, I’d be happy to take a look at your install. If you would like me to do this, please send login details via our secure contact form.