You actually don't even need to know where it's installed. You can use either:
open -a 'Keyboard Maestro Engine'
or
open -b com.stairways.keyboardmaestro.engine
and macOS will launch it wherever it is.
This is helpful if you don't know where an app is -- which is often true now if you are trying to launch one of Apple's own apps which look like they are in /Applications/ or /Applications/Utilities/ but aren't really, or if you are looking for an app like 'Screen Sharing.app' or 'Archive Utility' which has never been in an easy-to-find/remember folder.
That being said, @gglick's suggestion has one advantage: if there are multiple versions of Keyboard Maestro installed (for example, if you have a clone of your boot drive attached) the open command can get confused sometimes, so using the specific path as they recommended would ensure that you are opening the one you'd expect.