Helloooooo. I have a big problem.... I'm using a L293D Shield on my Arduino Mega to control 3 motors. But, when i launch the program, the motors make a little noise but dont turn. When i put the battery (6V) dierctly on motors bounds, they turn perfectly. PLZ tell me what i can do ..... 
Use a battery with higher voltage. L293D drops 2-3V from your power supply so if you want 6V at the motors you need more like 8-9V into the L293D Shield. But don't try one of those little PP3-style 9V batteries, they can't supply enough current. Something like 6 x AA cells would probably work.
Steve