Hey Andy, welcome to the forums!
...Unfortunately, you can't download it at the moment. The reason for this is that it is being de-trekked to avoid legal issues in the future; it will instead have content (Technologies, empires, minors race, and ship models) that are not Trek related. Once this work is complete, a link to download the game will be posted. For people to download. We'll send an email or something to make sure people know, but there will definitely be a thread on it as well so keep checking. I doubt it'll be very long now.
Sorry I can't give you any good news about Birth of the Empires, but if you want to play something while you're waiting, check out Supremacy isn't as complete as BotE purely because BotE has been under development for almost four years as opposed to Supremacy's one year, but Supremacy still has it's Trek content and is pure graphical eye-candy.
