The post build event on the mojoPortal.Features.UI project should be copying the mojoPortal.Features.UI.dll, mojoPortal.Features.Business.dll, and mojoPortal.Features.Data.dll files to the Web/bin folder.
Is that happening, do the files exist there? If not you may want to look at the post build event in the project properties for mojoPortal.Features.UI.
The build profile may also be a factor, we have differnet build profiles for each of the supported data layers that appear in the dropdown in VS, ie Debug and Release are for SQL Server and Debug - SqlCe and Release - SqlCe are for SQL CE, but I'm not sure if those could have become hosed up when you removed the projects for the other databases.
Please also tell me how you obtained the source code, the supported way is with TortoiseHG, other ways such as the download links on the source code tab at codeplex often produce weird results. Unfortunately I can't do anything to remove those download links on codeplex.
Hope that helps,