Apparently there are some issues with getting the correct Microsoft.Web.Atlas.dll and other annoyances when downloading the December bits. Jonathan Hawkings has a good summary of what to do on the ASP.NET forums, and I reproduce it here:
It looks like there is a replication issue with the ASP.NET ‘Atlas’ December ’05 release family on MS Download Center (download.microsoft.com). The December bits were originally released with the October release family (a download center deployment term) and they suspect this lead to the October bits continuing to be downloaded. The download center folks have since re-release the ‘Atlas’ December bits with a new release family.
Please try the steps below to clear our prior ‘Atlas’ versions from the IE cache, VS project templates etc.
1. Make sure you are running ASP.NET 2.0 RTM, Visual Studio 2005 RTM or Visual Web Developer RTM … ASP.NET ‘Atlas’ will only run on the RTM releases. Beta2 will not work. If you don’t have a 2005 RTM release you can download the RTM release of Visual Web Developer from http://msdn.microsoft.com/vstudio/express/vwd/download/. Uninstall prior beta 2005 releases.
Verify your version numbers by choose Help -> About in Visual Studio 2005 / VWD
Microsoft Visual Studio 20005 8.0.50727.42
Microsoft .NET Framework 2.0.50727
Close Visual Studio / Visual Web Developer
2. Clear out your Internet Explorer cache
Internet Explorer -> Tools -> Internet Options -> Delete Files
Close Internet Explorer
3. Delete older releases of the ASP.NET ‘Atlas’ VSIs
C:\Documents and Settings\username\My Documents\Visual Studio 2005\Templates\ProjectTemplates\Visual Web Developer\CSharp\ASPNETAtlasWebApplicationCS.zip
C:\Documents and Settings\username\My Documents\Visual Studio 2005\Templates\ProjectTemplates\Visual Web Developer\VisualBasic\ASPNETAtlasWebApplicationVB.zip
C:\Documents and Settings\username\Application Data\Microsoft\VisualStudio\8.0\ProjectTemplatesCache\Visual Web Developer\CSharp\ASPNETAtlasWebApplicationCS.zip
C:\Documents and Settings\username\Application Data\Microsoft\VisualStudio\8.0\ProjectTemplatesCache\Visual Web Developer\VisualBasic\ASPNETAtlasWebApplicationVB.zip
4. Start Internet Explorer and download the December VSI
a. Navigate to http://atlas.asp.net/ (which redirects to http://www.asp.net/default.aspx?tabindex=7&tabid=47)
b. Click the link “Atlas Blank Project VSI (December release)”
c. Click the link “VSI file for creating a blank ASP.NET "Atlas" Web site”
d. Click the link “I Accept” (the link should point to cfab4430-426d-4a04-954b-1acbf852e800/ASPNETAtlas.vsi )
e. Choose “Open”, Next, Finish, Close to install the December VSI
5. Start Visual Studio 2005 / Visual Web Developer 2005 and create a new ‘Atlas’ Web site
a. File -> New Web Site -> ASP.NET ‘Atlas’ Web Site
b. The Readme.txt should contain “Date: 12/19/2005” which is December 19, 2005.
6. Read the ASP.NET ‘Atlas’ Technology Preview December 2005 release summary change list at:
a. Navigate to http://weblogs.asp.net/atlas/archive/2005/12/22/433869.aspx