We have moved the xBIM source to GitHub to make things easier for you.
The best way to work with xBIM is to use the Nuget distribution packages. Version 4.0 has now been released and will be stable for the foreseeable future, we intend all changes to be backwardly compatible. If you just want to programme with IFC then all you need is Xbim.Essentials, if you want to do geometry operations then there is Xbim.Geometry. For viewing we have Windows and WebGL platform support. Very shortly we will move from pre-release to stable release a BIM query language and enhanced support for COBie, including validation.