Try changing the resolution & cycle settings of your laptop. Projectors have (and you must know) resolution parameters or limits. Refresh rate Hz cycles also must match. I would imagine that if the external video is not showing up in System Preferences:Displays window you have either a resolution/cycle matching problem OR a connector/adapter problem.
There is a "detect displays" button in System Preferences to use if connecting the cables/adaptor doesn't automagically find a second display.
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