Do you see anything in Disk Utility? Or any other disk reading app you might have?
Nope. This is a 'feature' of the GoPro - it just doesn't mount as an external disk (unlike eg my Garmin). Apparently older models did, but not more recent ones.
If you've got another device that takes SD cards and uses USB mass storage, you could try using that as a card reader.
Alas, I don't have any other devices that use micro SD cards.
I guess the simplest solution will be to get myself down to PC World...