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How are you connecting your projector (Thunderbolt, RCA, S-Video, HDMI?)
In System Preferences:Display does the projector show up and have a profile? It should. You may have to change the resolution/refresh rate (signal) of that profile in order to get the video to show up. Depending on your connector you might not be able to choose a compatible resolution and refresh rate (Study the information included with your projector) . . . in that case you might need to use a different adaptor/connection.