Refering to bridel.org website on drag and drop (using the cats) how does one insert a button into a movie clip?

I am refering to the following:


"Here I have placed a transparent rectangle within each
cat Movie clip and made it a button by making "Is a
symbol" equal to "Yes" and "is a button" equal to "Yes".
The actionScript is then placed in the "Actions"
field of the new button."

I am sure that it is easy.
Thank you