You can use 2 DC servos that run on 5V and up to 4 DC motors or 2 stepper motors (or 1 stepper and up to 2 DC motors)

No, at this time it is not possible to stack the shield or otherwise connect it up easily to control 4 steppers, for example.

Is the LED lit? The Stepper and DC motor connections wont do a single thing if the LED is not lit

Don't bother writing up uploading code or wiring up motors if the LED doesn't light up

The LED indicates motor power. If it is not lit, then the DC/Stepper motors will not run. The servo ports are 5V powered and does not use the DC motor supply

Please read the user manual for information about appropriate power supplies

Not really, its meant for larger, 6V+ motors. In theory you should be able to get it working with 3V motors but I have no information on how to do so or whether it will work

Please read the user manual for information about appropriate power supplies

Motors take a lot of power, and can cause 'brownouts' that reset the Arduino. For that reason the shield is designed for seperate (split) supplies - one for the electronics and one for the motor. Doing this will prevent brownouts. Please read the user manual for information about appropriate power supplies

Try soldering a ceramic or disc 0.1uF capacitor between the motor tabs (on the motor itself!) this will reduce noise that could be feeding back into the circuit (thanks macegr!)

All 6 analog input pins are available. They can also be used as digital pins (pins #14 thru 19)

Digital pin 2, and 13 are not used.

Digital pin 11: DC Motor #1 / Stepper #1 (activation/speed control)
Digital pin 3: DC Motor #2 / Stepper #1 (activation/speed control)
Digital pin 5: DC Motor #3 / Stepper #2 (activation/speed control)
Digital pin 6: DC Motor #4 / Stepper #2 (activation/speed control)
These pins are in use only if the DC/Stepper noted is in use

Digital pin 4, 7, 8 and 12 are used to drive the DC/Stepper motors via the 74HC595 serial-to-parallel latch
These pins are in use if any DC/steppers are used

Digitals pin 9: Servo #1 control
Digital pin 10: Servo #2 control
These pins are used only if that particular servo is in use

The DC/Stepper motors are NOT connected to the Arduino directly. They are connected to the 74HC595 latch which is spoken to by the Arduino. You CANNOT talk directly to the motors, you MUST use the motor shield library

You can try reading this nice overview written by Michael K

The analog pins (analog 0-5 also known as digital pins 14-19) are broken out in the bottom right corner.

Pin 2 has a small breakout since its the only truly unused pin

The remaining pins are not broken out because they could be used by the motor shield. If you are sure that you are not using those pins then you can connect to them by using stacking headers when assembling the kit or soldering onto the top of the header with wires

Make sure you have installed the AFMotor library

Download the latest Arduino library file and uncompress the folder. Place the folder AFMotor into the Arduino/hardware/library folder. That is, find your Arduino install folder, then open up hardware & library and drag AFMotor into it. Inside AFMotor should be a AFMotor.c and AFMotor.h file

The stepper motor library step() routine does not have the ability to run both motors at a time. Instead, you will have to 'interleave' the calls. For example, to have both motors step forward 100 times you must write code like this:

for (i=0; i<100; i++) {
motor1.step(1, FORWARD, SINGLE);
motor2.step(1, FORWARD, SINGLE);
}

Most people buy motors from surplus shops and no motor will make everyone happy

However, since its a popular question, I suggest buying motors from Pololu (DC Servos, DC motors) or Jameco (all sorts!) As well as the many surplus webshops.