To change the current library you are viewing, click on the drop down in the menu bar of the Reports tab. All of the libraries available to you will be listed here. Click on the specific library you want to see to change to that library.
In MITS Report, libraries are used to as a way to organize individual reports into meaningful collections. To start out, all users have access to the Common library. You will also likely have a personal library where you can save any reports you create or modify.
Additional libraries can be created by users with sufficient permissions. The ability to see and to save into those libraries are independently controlled. It may be possible for you to see a library but not be able to save new reports to that library. This access is controlled by your MITS administrator.