I'm not quite sure how it works for Steam, but usually a launcher appears first, with several options. The editors are one of them.
If that is not the case for Steam-version there are still two possibilities:
1. You have to start them manually. If you have used the standard installation path for Steam you should find them in: "C:\Program Files\Steam\steamapps\common\Galactic Civilizations II - Ultimate Edition\Twilight\Tools".
2. You have to download them separately. In this case you will find them in the tools category in your Steam-library. You have to download and install them from there.
I hope this helps.