I want to create a Post and Rail Fence, often used in a rural environment here in Australia.

You don't need the Furniture Library Editor to create new furniture. For your fence you could create a nice fence by combining multiple resized Box objects. Variations can be made by using stretched cylinder objects.

You should get the geo models from https://sourceforge.net/p/sweethome3d/d-models/299/ which are very useful for creating furniture details. For example to create different post tops. Or put a Sphere halfway in the end of a cylinder to get a nice rounded top.

Once you have all your parts combined you can export them to a single object which can be imported in the Furniture Library or just used as you created it.

The Furniture Library is mainly to group multiple furniture objects into a single Library for easy import or to create a collection of your custom made furniture. It's also important for adding and changing a few properties for objects that are specific for Sweethome3D and can't be added to a stand-alone object. You can, for example, change the cut-out made by windows, doors, and staircases. The Furniture Library allows to add these features.