You have to copy the text of the txt file you download from the editor and paste it on the construct 3 layout.
For details step by step guide watch this video.
This Editor is specifically designed to make buildings and levels for construct 3 Game projects. it's easy to used and implement in games.
However If you want to make complex designs like Weapons, cars, ships, planes, monsters, characters etc. I recommend using blender or any other 3D designing software that export in .gltf
Use the kindeyegames 3D object plugin to import it in your construct 3 projects.
I want to thanks to kindeyegames for making such a great plugin for Construct 3. That helped me build a useful 3D building Editor for construct 3 for making FPS Games
https://azeemdesigner.itch.io/3d-buildings-and-level-editor-for-construct-3
it would help me alot if you can recommend this tool.
thanks