Updated Answer:
Microsoft just released their "web deployment projects" for visual studio 2010.
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=711a2eef-b107-4784-9063-c978edc498cd#AdditionalInfo
After adding a web deployment project to your website (right click on the website name, click "add web deployment project") you can then build the package and use msdeploy to deploy your application.
Old answer:
A "web site project" cannot be packaged with MSDeploy through the GUI (at least not in the RC). You must convert your website into a "Web Application Project". Then you will have the full publishing/msdeploy capabilities.
Here is a link on how to convert a website to a web application project: http://gurustop.net/blog/2008/08/03/converting-vs-2008-website-to-web-application/
Once you convert to a web application project in Solution Explorer right click on the project name and click "Build Deployment Package".