Hi,
mojoPortal uses Web Application Projects and Class Library projects. Historically, these project types have not been supported in the Express versions of VS. There are 2 different project types in VS for web projects, they are Web Site Projects and Web Application projects and historically, the VS Express versions only supported the Web Site project type.
However, according to this post on Scott Guthrie's blog, they are adding support for Web Application and Class Library project types to the Express versions of VS in SP 1. They have a beta of it available here, I have not tried it myself and one person told me they had problems with blue screens trying to use it so I leave it up to you if you want to try the beta or not. But at least in theory it should work with mojoPortal, especially after the final release of SP 1 comes out.
Best,
Joe