Oh okay, I've made an error then. I guess I thought I would be able to start playing it on my PC and then later play on my Mac (which is newer/better) when it became available as long as it was tied to the same Steam account. I haven't downloaded the beta (or anything) in PC mode but I did generate a key and link it to my Steam account (I see it in the library but it's marked as 'not available on your platform') - do I need to contact support directly or should I be okay later?