Attain a copy
Bona Tile 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.
What is it?
The tile editor is a small Unity Editor extension, adding the possibility to create tile sets and then paint mapsegments in the scene view that looks like old othographic top down 2D games. It was initially intended to mimic the looks of old NES/SNES games but depending on the textures used and their import settings into unity this is not required.
What more is needed?
This plugin comes with example textures for you to learn how to use the editor. These tilesets are (in my own opinion) quite ugly for any real projects and are only included to serve the tutorial. Proper texture to use are needed. I suggest taking a look at OpenGameArt.org for free-to-use tilesets. I must point out that most tilesets found need some minor editing to take advantage of the layered system this plugin uses.