I'm looking for an editor that has the help from
http://www.hibernate.org/hib_docs/nhibernate/html/mapping.html
built in, and allows simple editing of the XML files in a GUI fashion. I realise there's CodeSmith and MyGeneration, but from what I remember these only go one way, and don't allow editing existing HBM files.
I don't believe there is an editor for Hibernate/NHibernate mapping files that has context-sensitive help. If you are a .NET developer (using NHibernate and Visual Studio .NET) you can edit the mappings with IntelliSense by registering a schema document: see Using NHibernate with Visual Studio .NET for instructions.
Another option to consider is using the NHibernate's Fluent API. This replaces mapping files with strongly-typed C# code.
There is another option with Visual Studio. If you have Resharper you can download the plug-in for NHibernate mapping files. This solution has worked out perfectly for me. You can read more about the plug in here: http://nhplugin.lieser-online.de/. Hope this works for you.
Why not use ActiveWriter to generate the mappings? You can always edit them afterwards. AW generates the model / mapping for both NHibernate and ActiveRecord. The editor runs WITHIN Visual Studio.NET and you can do drag-drop via server explorer.
The best option I've come across is is IDEA's Hibernate/JPA editor. IDEA is not free, however.
Have you tried Hibernate's own Eclipse plug-in named Hibernate Tools for Eclipse and Ant?
http://www.hibernate.org/255.html
Even if you dont have expirience with Eclipse and its add-ons it should be fairly straightforward to install and use their addon:
Just download it, unzip it into the eclipse directory, and fire up the IDE. What you need to do next is to open up the Hibernate perspective.