The SIM slot is located on the front left (as you are looking at the side with the lights and the reset button) behind the plastic cover. To access the SIM slot on the device you must remove the plastic cover with a 2mm or 5/64″ hexagon Allen wrench. Install the SIM card with the gold contacts facing down and the cut-out facing to the right. The GX400/440 also includes a SIM lock-down feature to prevent the SIM from moving during extreme vibration. The cover has a tab to mechanically secure the SIM in place during extreme vibration. When replacing the cover, first place the front into the lip and push back to make sure the four holes are aligned before screwing the bolts back in place.