This is taken straight from the help in flash on "working
with common libraries"
Working with common libraries
You can use the sample common libraries included with Flash
to add buttons or sounds to your documents. You can also create
custom common libraries, which you can then use with any documents
that you create.
To use an item from a common library in a document:
Select Window > Other Panels > Common Libraries, and
select a library from the submenu.
Drag an item from the common library into the library for the
To create a common library for your Flash application:
Create a Flash file with a library containing the symbols
that you want to include in the permanent library.
Place the Flash file in the Libraries folder located in the
Flash application folder on your hard disk.
Note: The Libraries folder is located in the
application-level configuration folder, one of several
configuration folders placed on your hard drive when you install
Flash. For the location of configuration folders, see Configuration
folders installed with Flash.