Attain a copy
Bona World Editor is distrubted through Unitys asset store and can be bought there. This is (at the moment) the only way you get your hands on it.
Once the plugin has been downloaded into your unity folder, it should be ready to use. The first step is to create Asset categories and Asset sets. These are collections of models, grouped together in a sense you find logical. Asset Sets and Assets categories can be created from the menu Assets->Create->Bona World Editor.
What is it?
The World editor is a unity editor extension for building world easier. It adds a new Editor window where prefabs are presented to you and you are quickly able to place lots of them in the scene without the need to drag and drop prefabs form the projects window or duplicating already present in-scene. The editor also helps with randomized rotation and scaling to create less sterile looking scenes..
What more is needed?
This plugin is distributed as-is without any assets to use. In order to to use the world editor some models needed. These can be found easyily. I suggest first taking a look at Unity's own asset store for a plethora of great models. Another source of geat models can be OpenGameArt.org
. The editor is of course available for any assets you create yourself.