Depending on what edition the Zen Thermostat is, there is a supported iOS and Android app.
The Zen Thermostat Wi-Fi edition is supported by the Zen Thermostat WiFi App.
The iOS Zen Thermostat WiFi App can be found here
The Android Zen Thermostat WiFi App can be found here
The Zen Thermostat ZigBee is not supported by the Zen Thermostat WiFi App. To control the Zen Thermostat ZigBee edition remotely you would need to integrate a home automation system such as Samsung SmartThings or Securifi Almond Hubs.
To verify the edition of the Zen Thermostat simply take the thermostat off of the backplate, on the back of the thermostat there is an indicator sticker with the serial number as well as either a MAC number or ZID number. If the Zen has a MAC number it is a WiFi edition thermostat. If the Zen has a ZID number is it a Zigbee edition thermostat.