click on the Specifications tab
it has 2 different motor types listed: "green" and "yellow"
the green has the following stats listed for 6 volt supply:
No-load current(6V): 71 mA
Stall current(6V): 470 mA
This means with no load (IE holding your robot in the air with the wheels not touching anything), the motor draws 71 mA.
Stall current means that if you were then to grab the wheel and prevents it from moving it would draw 470 mA.
Your motor driver needs to be able to handle the strongest current draw of your motor without overloading. My understanding is that to do this it needs to be able to handle the stall current of the motor that is connected to it. So for this motor you would need to handle 470 mA per motor. The chassis has 4 motors.
If you were to use the following controller: http://www.adafruit.com/products/81
It has 4 channels that each can handle a maximum of 600 mA per channel (1200 mA peak -per spec sheet -> non-repetitive, t ≤ 100 µs).
Since 1 "green" motor needs 470 mA, you could connect 1 motor per channel with out going over the maximum current per channel.
There is 4 channels and 4 motors, so this would work.
if you were to use the following controller: http://www.amazon.com/DFRobot-Motor-Shield-for-Arduino/dp/B006D85PAS/ref=sr_1_4?s=electronics&ie=UTF8&qid=1323643740&sr=1-4
It has 2 channels that each can handle a maximum of 2000 mA per channel DC (see spec sheet for peak info).
Since 2 "green motors need 940 mA, you could connect 2 motors per channel with out going over the maximum current per channel.
There is 2 channels and 4 motors, so this would work.
Now for the "yellow" motor:
No load current (6V): 160 mA
Locked-rotor current (6V): 2800 mA
The first controller can't handle 2800 mA per channel. therefore will not work for "yellow" motors.
The second controller can't handle 5600 mA (2800 mA x 2) per channel. Therefore will not work for "yellow" motors.
So the conclusion I have is this: are you getting the "green" motors or the "yellow" motors?
"Green" = you can make either work
"Yellow" = neither can handle the peak current usage, so your controller might burn out if your robot got stuck.
Does that help?