Hello,
If you take a look at the transport bar in Notion 6, there is an icon of the metronome. You will want to click that icon and choose to play the metronome during playback, recording, or both. If you are still having a problem, you will want to remove the app from your Applications folder and download/install it once more from your my.presonus.com account. Make sure that you download a new version as the metronome sound file is bundled inside of the app.