high low pin question

im using an arduino mega 2560 trying top use pins 3 and 4 for a motor one would be high and the oter low for forward then reversed for reverse allthough i cant get it to move at all unless i attach it to 5v and grnd can i not use two pins for motors like this on mega?
i tried this code and this doesnt work at all

 int motorPin1 = 3;
int motorPin2 = 4;
void setup(void)
  pinMode(motorPin1, OUTPUT);
  pinMode(motorPin2, OUTPUT);

void loop(void) 
    digitalWrite(motorPin1, HIGH);
    digitalWrite(motorPin2, LOW);

What is the current draw of the motor?

Unless your motor is TINY (like the size of a cellphone vibrator motor) you can't run it directly from an Arduino pin. To run a DC motor in both direction you need an H-Bridge motor driver. The easiest way to wire one up is to get a Motor Shield which plugs on top of an Arduino UNO, Leonardo, or MEGA. If you are using other shields which have conflicting use of the pins you can get a stand-alone motor driver based on the L298N chip for under $3 on eBay.