Ok wait, I found terminal now, I didn't realize it was an application (and yes it is in the utilities folder). Awesome, now I am in the toribash directory.
Ok, so the last step I may have just left out one word/command. I typed in "ffmpeg -i screenshots/123_screenshot_%04d.ppm -sameq -t svcd -aspect 4:3 123_movie.mpeg" exactly as you said, only with "123" replaced witht the correct screenshot numbers. The terminal is telling me that the "ffmpeg: command could not be found."
Almost there...