I was also having the same problem just solve it by adding the explicitly the bin folder in path
instead of "NHibernate.dll" use like below :
Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().Location)+ "\\NHibernate.dll")
it will now look for the "NHibernate.dll" in the bin directory the same worked for me
i Hope it will work for you also