I recently bought the 1 month subscription to get access to the entire course content, but there are no source files that are referenced in the journal of the lessons.
In the IDE area, there is catkin_ws with build, devel, and src folders. Build and devel have contents, but src does not for any lesson I launch.
That is normal. When you start a new course the workspace will be empty. Then, as you progress through the course and complete the different exercises, you will populate your workspace with different packages / files.