Entering # symbol in chord names above the stave

There seems to be a problem when entering chord names above the stave on the latest MacBook Air. Using the letter 'b' for a flat works perfectly (so my basic technique must be right) but the hash sign doesn't register at all - it's as if the key hasn't even been pressed. The hash sign is accessed by pressing 'alt' and then 3, though whether this has any relevance I don't know. Has anyone else experienced this - or better still, found a solution?