What is the file tree in the IDE?
The file tree is highlighted in the image below, in the red rectangle.
File tree missing
If your file tree is not showing up in the IDE, first make sure you are on the most up to date version of the IDE. You can check this in the file menu under Learn IDE
--> Check for Update or
File --> Check for Update.
If you are on the most recent version of the IDE go to Packages > Learn IDE Tree > Refresh. If the refresh doesn't work immediately, then quit and restart the IDE.