Your 4-in-1 multisensor operates on 4 AAA batteries (included). To insert them, begin by twisting the cover open on the multisensor as shown below:
Now insert 4 AAA batteries oriented according to the picture located in the battery holder (with the negative end depressing the battery spring), as shown:
Lock the multisensor main unit to its battery cover by aligning the Lock/Unlock Half-Dimple Markers under the “unlock” symbol and twisting the battery cover from the main unit in a clockwise motion until the Lock/Unlock Half-Dimple Markers form a circle under the “lock” symbol as shown below. You'll feel a "click" to let you know it's closed.