MiiCoffee Digital Milk Frothing Thermometer comes with 2 LR44 batteries in the package. You will need to install those after you receive the package.
Please follow this video.
If the thermometer won't turn on after you install the batteries please use a multimeter to measure the battery voltage. The full battery should be at 1.6V. The low battery is below 1.4V. Please replace the batteries if they reach low.