Now that you have a location to store libraries you are going to add a new library to the libraries stack.
Open libControlFormatter.rev Stack File
Choose File > Open Stack... from the menu.
Open the libControlFormatter.rev file that is located in the Homework Files folder of the distribution.
Change the mainstack
Once the libControlFormatter stack opens, open the Stack Inspector and change the mainstack property from libControlFormatter to libraries.
Save Your Work
Choose File > Save from the menu. This will save the libraries.rev stack file with the libControlFormatter stack addition.
Check Your Work
The Application Browser should now look like this (you may need to click the Refresh button again).