i just wanted to say THIS IS MY FIRST GUIDE, THIS IS AFTER THE KENNEN ATTACK RANGE NERF (575 reduced to 550).
ad kennen is a pretty good ad carry because
1. his w passive scales on ad
2. (i am not sure this is still true because of his nerf 575 to 550) he has great range
3. his e is great for initianting escaping and chasing because of the armor and mr and the movement speed
the runes i recommend are armor seals, mr glyphs, attack damage per lvl and just basic attack damage in marks, and finally for quintenses get armor pen. this runepage will give you durabilitie and damage.
there are a couple differnt masterie pages you can do but the one i use is getting all the attack damage mastries then to top it off get three in the cdr meaning 30 in the attack mastries.
Q: max at lvl 1 because the passive scales with attack damage. max by lvl 9
E: get at lvl 2 or at lvl 3 ( depending if you need the speed/armor and mr or not) max by lvl 13 beacause it makes kennen fast in and out of fights and give durabilitie.
R: max by 16 oblivously get 6/11/16
i recommend getting heal and flash. heal is basic and adds to his durabilitie and because of it all you can easy bait enemys with it up.
flash is really helpfull as kennens only escape is his e or stun. with flash you can flash ult into the enemy team and escape a losing teamfight.
you should be getting as many last hits as possible so you can get more powerfull items faster and allows you to get more durable as you level up you e and just basic stats