Fair Vote doesn't actually have a position on mandatory voting. I would have to say, though, that it's a double-edged sword. Not to be elitist about it, but a number of people choose not to pay attention, not to vote. That's their choice. Forcing them to vote would tend to engender some resentment. I'm not sure how you would force them to vote. Maybe fine them, as they do in Australia, if they don't cast a ballot.
I'd be more in favour of mandatory participation, where you have to show up rather than necessarily having to mark a ballot.