Reading your specs it’s kind of suprising that it’s even running. Your Geforce cards does have only 256MB what is below the minimal specs.
You could give us you steam display name, you should be able to find youself with this search:
http://steamcommunity.com/search/
So we can have a look if we even get any crash dumps. It could be that this is not the case if your macbook needs a hard-reset.