I found this good post in StackOverflow, I put it here simply to have it always present.
To enable it, try copying the following files into: C:\Program Files\Microsoft Visual Studio 9.0\Common7\Packages\schemas\xml, or into C:\Program Files\Microsoft Visual Studio 9.0\Xml\Schemas.
I'm not sure which is the right place and I put them into both of mine but it works for me.
nhibernate-generic.xsd(I could not find this file in the last version of the code of NHibernate
You can get the files from the source of nHibernate
Taken from here