Hey there! I admit, neither of us have a mac, so we'd be pretty miserable at directing you, but if it works like it ought to, if you hit the save button and select 'save to disc' it should give you a file you can transfer to any PC you want. Then 'load from disc' should let you load that into the game on any computer.
Some notables, you need to have the save loaded when you save to disc, as in actively playing. It doesn't download all of your saves, it will only create a new save to the current screen that you're on when you export it.
Cheers!