You can find the Micro-HID room in Heavy Containment; you will know it is the correct room when you see a large pane of glass between two doors. You will need a keycard with Armory Access Level 3 to open the door inside, and the Micro-HID will be sitting on the table.
From there, charge the Micro-HID when near an SCP. It will take about 6 seconds to begin firing; the ammo charge is eaten up quickly, so use the charge sparingly. You can also recharge the Micro-HID, which will pop the Overcharged Achievement.
The easiest SCP to kill with it is 096 - Charge up the Micro-HID, and SCP 096 will be unable to escape due to his long screaming animation or slow speed when non-hostile.