There are a number of ways to time visual and sound content in Keynote, depending on the particular slideshow being created. The quickest and easiest way is to use Record Slideshow from the Play menu.
timing video playback using record slideshow
- place the content on separate slides
- set Presentation Type to Normal: Inspector Document > Document > Presentation Type: Normal
- select the slide where the recording is to start.
- go to Play > Record slide show
- click the red button to start, then press the spacebar to advance through the presentation
- if a narration is to be added while recording select the microphone to be used in System Preferences > Sound > Input and adjust the input volume slider
- when finished, click the red button again, then click the X button top right to exit Record