Good question. The required HTML-Files come with the download. However, the program doesn't seem to find it. In the title it says "null" and you get an error when trying to open it in a browser telling you that the requested file can't be found. Unfortunately, I didn't find a way to make them appear by clicking on Actor/World. Seems to be a bug in 3.5.0. You can also find the Greenfoot API here: https://www.greenfoot.org/files/javadoc/