AD KENNEN IS HUGELY VIABLE.
80 games won in ranked with him (75% of games played with AD Kennen)

Starters:
boots + 3x health pots.

Then you need to build based on what their team is building. I usually rush Madred's razors (for farming) then go Phage, Zerker's greaves and finish Madred's up. Then wit's end, finish Frozen Mallet and then build a Phantom Dancer and an IE.

Skills: W,E,Q,W,W,R,W,E,W (choose on your own from here on out.)

Don't mid as AD Kennen, try to get bot with a support. Taric, Nidalee, Nunu, and Sona are all great supports for him. Try your own play style as well. Your E is usually used as an escape or an initiate, your q is to stun stack and tag opponents from a distance. And your ult is for team fights to disrupt and 1v1's to gain distance or initiate. or escape from other situations. good luck and happy playing sir.

Start Boots Three Potions
2 Dorans Blades or One if you are doing good
Wriggles
Zerkers
PD
IE
QSS
BT
LW
Sell dorans and wriggles later.

ok so i play AD kennen and i love him. but for some reason i cant play ap kennen. but my kennen build is what everyone calls AD Tanky. so 1st your gonna grab a dorans blade cause u need the little sustain remember with kennens W it deals magic damage so you wont be grabbing much life steal with this build.
now if you dont feel your strong in last hitting them minions with your free cost combination of skills after your dorans and boots(Attack speed) id get wriggles. if not just get the life steal scepter and then rush
>> ionic spark. (hp and nice attack speed with bonus magic damage)
>> Frozen mallet (or warmogs if you feel like it)
>> Atma's
>> Maw
>> finish with BT for damage. and that extra life steal.

this build will keep you tanky and dealing enough damage to shut people down. remember to use that bonus damage from your W on other champions as it deals alot of damage.

Yes, I can actually:

First item: Alt+F4
Second item: Uninstall
Third item: Run Eraser and use the 35 Guttman Pass erase on your previous LoL directory
Fourth item: Email Riot support and ask them to delete your account
Fifth item: Delete system32
Sixth item: Drop your PC off a tall building

You're welcome.

To refrain from being a total ass, here's a less sarcastic response:
Read some of the posts in this thread that are against playing AD Kennen.
It really should not be built unless you're in a full 5-man, as AP Kennen is a lot better, a lot easier to pull off, and a lot less likely to ruin the game for the other 4 people on your team.
Please think of the rest of your team before trying AD Kennen in soloqueue.