You can get to the preview either through the Expression website or the Download Center:

 

Expression site: http://www.microsoft.com/expression/products/download.aspx?key=blend2preview

 

Download Center: http://www.microsoft.com/downloads/details.aspx?FamilyId=69540337-B619-4A47-AC27-52D8AF3A7830&displaylang=en

 

Also available is the Update for the Expression Media Encoder Preview…this update ensures templates work properly with the latest Silverlight bits: http://www.microsoft.com/downloads/details.aspx?FamilyId=3A3C901C-C23D-4567-A76F-CC46CB113D1E&displaylang=en