Hmmm, try this code using the exp_system resource:
function rewardOnWasted ( killer )
local exp = getElementData(killer,"exp")
if exp then
setElementData(killer,"exp",tonumber(getElementData(killer,"exp"))+5)
outputChatBox("Experience gained +5! total ".. tonumber(exp)+5,killer,0,255,0)
else
setElementData(killer,"exp",5)
end
end
addEvent("onZombieWasted",true)
addEventHandler ( "onZombieWasted", getRootElement(), rewardOnWasted )