Notes
The original level editor is impossible to add, so instead I added a remake (which you can find at http://robbie.lucasiscool.nl), see the additional application. To use your custom level, replace the Robbie2D.xml file in Legacy/htdocs/kids.ambrasoft.nl/kids/ with the one you downloaded in the level editor.
Source
https://web.archive.org/web/20180917004748/http://kids.ambrasoft.nl/index.php?p=game&game_id=17