speech bubble - how do I add to iDVD or iMovie?

A few months ago someone showed me how to use iMovie and iDVD. I liked the additional feature they showed of being able to add "speech bubbles" to show text for individual frames/images. Unfortunately, I can't find or remember how to find this feature. Anyone able to help?
Many thanks

G4 PowerPC Mac OS X (10.4.8)