The LED on the front of your Extension Kit tells you what state your Extension Kit is in.  

The Extension Kit is working as intended.

The Extension Kit is not supplied with power.

This is usually because your Extension Kit is wired incorrectly. Make sure you double-check the wiring instructions in the tado° app.

The Extension Kit is pairing.

The Extension Kit is wired and powered, but is not paired yet. 

You need to pair the Extension Kit with the Internet Bridge.

If the Extension Kit is not running on the latest firmware, you may notice other blinking patterns on the device. Once the firmware has updated (this is an automatic process), the states should match those mentioned above.

More information:

How can I control my heating system when the radio link between the Smart Thermostat and the Extension Kit is lost?