Please make sure you are within Bluetooth distance of the device.
Please uninstall and reinstall the app. Then press firmly the round button at the front of the unit for 3 seconds. The unit should beep and the led lights will change colors. Once this has stopped and the unit is flashing green, then try connecting to the app once more.
If this does not work, try installing the app on another phone or tablet to see if there is a connectivity issue.