Executable not found

I dont own the game, but I have seen similar issues, and they're typically caused by the game being incorrectly marked as compatible with certain OSs. Although in this case perhaps you have an invalid download, so try verify the download, or failing that manually launching the exe from the SteamApps folder.

With the Gal Civ III alpha I know this has been an issue with people who do not have a 64-bit operating system. This is running on nitrous which supports just 64-bit platforms with DX11/mantle .

If you definitely have a 64-bit OS then as AlphaSite says you could try verifying the game cache in the game options.

I doubt I can be too specific here, but follow the directions in the email with the steam key, once that first part is activated, there are instructions available to enable the game in steam.

Yes, it sounds like you haven't followed all the directions in the email.  Please try to follow that and see if it helps.

I pre-ordered and didn't get any email. Was it only sent to a limited number of people?

It depends, when did you join? Because the prototype access is sent out once a week; if you missed the last batch you'll probably be waiting upto a week, but likely until  the 21st, so tomorrow. That's on the assumption they do it at the same time each week and this post was made on the day of or after the keys were distributed. 

Ah right that explains it, I think I joined just after the first keys were sent.

Yes, they are only sent out once per week so you would need to wait for the next batch.

The pre-order website should mention the "once a week" keys/NDAs are sent out.  It's very misleading with the "Play the Prototype Now" when I don't get to play the prototype 'now'. 

There must have been a fault with the download from Steam. The latest update fixed everything - I can now play the game.