The DA's button is a handy button. By clicking it, you will be shown a dialog box of all currently installed desk accessory's (supports Suitcase'd DA's as well). From this dialog box, you can open as many DA's as you'd like simultaneously, by command-clicking. Of course, you can open only one if you'd like.
The INDEX button gives you an index of FileFinder cards. Clicking on one will automatically take you there.
The "File" button will let you open uninstalled applications, documents, and stacks, or print out documents and stacks. It is a hierarchical menu.
To change the name of the FileFinder card, click on the current name. You will be asked to supply a new name.