The Wah effect pedal can be used for the rhythm guitar part Bob Marley is playing by pressing all the way down on your wah pedal. Using the clean channel of your guitar amp this should give your guitar more top end (treble).
The wah pedal can also be used during the lead guitar part that Peter Tosh is playing. You want to slowly press down on the pedal as you play the chord on the downbeat.
With guitar effect pedals less is always more, so try to be economic with your use of the Wah pedal when playing