[State 20, PowerAdd]
type = PowerAdd
trigger1 = 1
value = 5
Put this under statedef 20. If there is no statedef 20, create one for your character (you can copy it from common1.cns)
Edit: Ah, I noticed you want it only when walking forward, the code I provided add power if you walk backward too. If you want only for walk forward, replace "trigger1 = 1" with "trigger1 = vel x > 0".