Apologies for the delayed answer, illness cares not for forums.
If you want to use the map editor then yes you will need to download the Shogun 2 Ted in Steam Tools. However, unlike the Assembly Kit the Ted can be launched via Steam, or a shortcut if you wish to make one, like games on Steam.
As for launching the Assembly Kit, you have to unpack the archive (you can extract it anywhere you want really) and locate the executables for each tool. Fortunately I do believe, from recollection, that they are located in the Binaries folder within the .zip archive.