We've been hearing this question a lot and here's the answer:
If you already have access to SharePoint 2007 Beta 2 on the Microsoft Betaplace site, then you also have access to KN.
If you do have this access, once you get to the main page on Microsoft Betaplace, you can follow the link for Knowledge Network that can be found by following the “Other Files” link under the “Beta Downloads” node in left navigation bar. From here you can follow the instructions to download the actual files for the software and associated documentation.
If you don't have access to SharePoint 2007 Beta 2 on the Microsoft Betaplace site, then you won't be able to access and use KN yet. We will update information about broader KN availability in the coming months.