You create the Test Cases in the Test Plan Tab and link them to the requirements in the requirements Tab. Once the Test Caseis Ready..u change the status to ready and goto the "Test Lab" Tab and create a Test Set and add the test cases to the test set and you can run from there.

For Automation...In Test Plan ...create a new automated test and launch the tool and create the script and save it and you can run from the Test lab the same way as you did for the Manual test cases..

To answer your question...the test cases are stored in Test Plan Tab...or more precisely...in the TestDirector..Lets say Quality Center 's database

TD is now referred to as QualityCenter