I'm having an issue with getting the voice over tool in FCP to capture any sound through a Blackmagic Extreme HD card. I've tried it with a mic plugged in directly and through a Yamaha Mixer but it still doesn't work. Originally we thought it may be the card itself but we now have the same issue on a new Mac and new card we've just purchased.
Have you got a level coming in to the FCP from the mic? You should see this without having to record.
Make sure the input/source are correctly set in the Voice Over window, what these should be depends on your set up. Your timeline needs an in and out point (you have to set an area to of the timeline to record to), before FCP will record onto the time line. Not 100% but I don't think you can record VO if 'log and capture' or 'Edit to tape' windows are open.
Have you tried recording audio in other programmes? This is more to check that the audio is making it through the mic and mixer alright.