NHibernate could not be found

If you see this kind of compile-time error even when you already add NHibernate to your project reference...

The type or namespace name 'NHibernate' could not be found (are you missing a using directive or an assembly reference?)

...Just change your project target framework. Click Project > YourProjectNameHere Properties > change Target Framework from .NET Framework 4 Client Profile to .NET Framework 4