there should be a nut on the bass of the wiper pivot...if not...i hate to say it but it might be underneath...if so...its a b!tc# to get to...
but i think you are refferring to the screw...pull the wiper arm out so that its sticking straight out and tighten the screw at the base of the wiper arm
and if that doesnt work...grab a 12 pack of beer and strart fishing around its gonna be a journey