Coming back to the main topic - driving stepper motor with this Monster Moto Shield. I have been getting answers from these forums for years now and its time to give something back 
Polyglot's answer is right on the target by the way.
I have had some past designing of stepper motors and their drives and below are some equations calculating the power supply for stepper motors (from stepper motor manufacturers) :
A_max=A_stepper∙1,4∙ number of phases
V_max=V_stepper∙x
V_(max2)=32∙√L∙0,8
Above the A_max is maximum current per phase that the motor can take 100% without damaging it, V_max and V_max2 are direct current voltages that can be fed to the stepper motor. Notice that there are actually two V_max equations, but they basically give the same value. x is a variable between 5...20, it will give more speed to the motor when using higher value. In the equations numbers 1,4, 32 and 0,8 are empirically derived numbers so don't change them. L is the inductance in mH.
So coming to this topics motor example the motor's specs are the following:
Current Rating 2.8 A <- this current is per phase, this stepper has 2 phases
Frame Size 56.4 x 56.4mm
Holding Torque 1.26Nm
Number Of Wires 4 <- 2 phases
Resistance Per Phase 0.9 Ohm
Shaft Diameter 6.35mm
Shaft Length 19mm
Step Angle 0.9°
Stepper Motor Type Hybrid, Permanent Magnet
Voltage Rating 2.5 V
Inductance per phase 4,5 mH
Calculating power supply (and stepper motor drive) for this stepper motor:
A_max=2,8 A ∙ 1,4 ∙ 2 = 7,84 A
V_max= 2,5 V ∙ 12 = 30 V
V_(max2)=32∙√(4,5∙0,8) = 60,7 V
Notice that when I put to the V-max equation 20 instead of 12, the max voltage is 50 V, so the V_max equations do not differ that much.
When using Monster MotoShield, in the shield's data it is said that maximum voltage is up to 16 V, so you cannot actually use a 50-60 V power supply. You can use a quite common 15 VDC power supply @ 8 A to use the motors full potential.
One more equation to actually calculate the maximum speed that the motor can reach (its physical properties taken in mind) with the given power supply:
RPM=U/(L∙2∙I_phase∙y)
y is the number of steps in a 360 degree full rotation. This motor has 0,9 degree steps so y = 400. I_max_phase is the phase current of the stepper (given in specs).
So RPM for this particular motor:
RPM = 15/(0,0045∙2∙2,8∙400) = 1,38 rpm
Notice that if you'd use a different power supply with different driver that can take 60 V the speed would be 5,56 rpm.
TAKE THESE EGUATIONS "AS IS". The have worked for me so far but there isn't a 100% guarantee that they would fit to every single stepper motor.
Btw would someone please give the right code fo using this Monster Moto Shiled and finish the topic?
Cheers!