AFAIK, If you have preordered, you'll be provided with a beta cd key by email when the next major beta version is released (currently beta v4, which is scheduled for release this coming thursday). You can then put that key into StarDock Central and download the beta when it is made available. It is important to note that there is a limited window of time during which the beta is available to preorder customers, so if you miss one (such as the current beta v3), you will have to wait for the next major beta version to get it. This limitation is simply so that the programmers are not flooded with bug reports (from new downloaders) of issues that have already been fixed.
Or if you are a TotalGaming.Net subscriber, you can simply downlaod the beta at any time (regardless of when a new major version is released), just as you would other TGN games.
