Go to Libraries, then click on "add JAR/Folder" and add the following jars:
/usr/bin/greenfoot/lib/bluejcore.jar
/usr/bin/greenfoot/lib/extensions/greenfoot.jar
I can't find them.
I have Windows 10 Pro, and I found them here:
C:\Program Files (x86)\Greenfoot\lib\bluejcore.jar
C:\Program Files (x86)\Greenfoot\lib\extensions\greenfoot.jar