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Does the motor shield use IOREF? Probably not. Just cut that pin off so the motor shield can stack on top of the USB shield. The other pin there does nothing so it can always be cut off with no impact.
If it does use IOREF - the documentation specifically says that it can adapt to different Arduino input/output voltages, then cut it but put a jumper on top to join that pin on the upper header to the appropriate power pin - 5V in the case of the Mega.
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Carefully solder 6 wires to the top side of the female ISCP header on the USB shield.
Terminate them on a 6 pin female header and plug that into your Mega.
aqmal_duino:
Could you help me to confirm the IOREF pin? I'm quite new in this thing... I use a L298P R3 motor shield..Many Thanks
That doesn't tell us enough. Please provide a link to where you bought it. Lots of shields use that chip, including the actual Arduino-brand motor shield. That particular shield does have IOREF connected, but it appears to only be a protection diode for the IN2 and IN3 analog inputs on the board. It will work without IOREF connected.
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