Some tips:

Aim at the shadows or rather infront of the shadows, not the planes themselves.

Use the distance between the plane sprites and the shadows to detirmine what angle to use. This of course comes with experience more then anything.

The closer a plane is to your ship the higher you should be firing even if it is below default height.

Check out these guides:

http://nf-guides.com/aa.htm - Will show you the basics of AA

http://www.opennavyfield.com/smf/index.php?topic=421.0 - A guide to KM aa ship setups

Not really sure if theres anything else i can really tell you to help you out here but feel free to PM me ingame if you see me. I am normaly not on my main account, rather BlackProject or Blitzed216.

and make sure you know the gun's golden angle as well...

And the most important thing:
use Manual FCS if you aren't already 

Tnx guys!

the golden angle on the lvl 30 km aa guns is 36

and i suggest u play gb , if u are aa-ing .

don't bother with aa in blitz and the aa mission is joke ...the second is a bit better ...but u gain nothing from it.

good luck , practice practice practice

Hi General:

Don´t worry if your AA gunners don´t shoot as fast as required for that mission, they will better.

As V2C said, aim for the shadows and us the guides that he linked to know where to aim.

Just keep in mind the golden angles of the three mian AA guns for KM:

2 x 3.46"/76 SK C/32 (LVL 30) -> 36 Degrees
2 x 5"/61 KM 40 (LVL 42) -> 32 Degrees
2 x 4.1"/65 SK C/33 A (LVL55) -> 32, almost 33 degrees

The best AA guns for me are de 5", but as this use more space than 4,1" I use them only in BB´s. and mount the 4.1" for AA duties.

I hope this can help you. 

One thing I did that helped me get a feel for AA was go into one of the training scenarios, launch a scout and try to shoot down my own scout.

That may not be helpful to you in a DD, but maybe be helpful to others.