From Offensive-security.com

Supported Firmware

• To see the firmware that is loaded on your card, you will need to do:

cat /proc/driver/aironet/eth2/Status| grep Firmware

Tested and verified firmware that will work with kismet are:

Firmware Version: 4.25.30
Firmware Version: 5.30.17

Please report other firmwares which will or will not work with kismet. If you have a firmware version that does not work with kismet:

N.B.: You will need to register here before you can start downloading any of the firmwares.

Checking to see if it is actually in rfmon mode

• To see if you are in the correct mode (rfmon), you will need to do:

cat /proc/driver/aironet/eth2/Config| grep Mode

You may get various output but anything with rfmon written in the Mode: line shows that it is in rfmon mode:

Mode: rfmon

OR

Mode: yna (any) bss rfmon

Kismet basic guide

(Partial information copied from kismet's site)

         cisco_wifix     Aironet 340,350     Linux       Kernel 2.4.20+, CVS  
         http://sourceforge.net/projects/airo-linux/  
         Capture interface:  'ethX:wifiX'

All you need to do is add cisco_wifix as driver and specify ethX:wifiX (replacing X with the actual number the aironet cards are sitting on)