If you had a TypeInitializationException when you are trying to do this:

NHibernate.Cfg.Configuration conf = new NHibernate.Cfg.Configuration();

Make  sure that your app.config is well formed and is coherent with the 
nhibernate-configuration-2.0.xsd shema.

In my case, I had my config file ending like this:

<add key="hibernate.connection.connection_string"
      value="User ID=blabla;Password=blabla;Initial Catalog=blabla;Data 
Source=blabla"
    />
    <add /> <-------------- Here is the mistake
  </nhibernate>
</configuration>

The shema says that in the "add" node, the key and value attributtes are 
mandatory.