Make sure the key you have entered is exactly as shown in the receipt/download page (It is case sensitive!) and that nothing extra is being added when you pasted, such as spaces or 'v' when you're pasting the key in.
Recent community posts
Make sure you have unzipped the folder you downloaded from here; it won't work if it's still zipped. To do that, just right-click and unzip. The game should run properly once you've done this. If not, let us know.
There will be an in-game messaging system, so you can contact any other players you wish to invite before or after you have a session running. Phrase chat strictly applies to open games; you get to choose between voice and text chat while setting up a private game.