All Z-Wave devices have to be reset before they can join a new gateway/controller.
To reset it, according to the manual (with added parts for the Almond+) do the following.
Tap on Add Sensor, then Remove in the LCD UI of the Almond+.
Make sure the Almond+ is no longer than 1m/6 feet from the thermostat.
Press the Menu button on the thermostat.
Scroll down to the Z-Wave Install menu option and press the Select button.
Press the Yes button to remove/reset the thermostat.
The orange light on the thermostat should blink and then turn solid once it has been removed/reset and the LCD UI on the Almond+ will show a popup with a message about an unknown device having been removed.
To add the thermostat to the Almond+, do the following.
Tap on Add Sensor in the LCD UI, then Add.
Make sure the Almond+ is no longer than 1m/6 feet from the thermostat.
Press the Menu button on the thermostat.
Scroll down to the Z-Wave Install menu option and press the Select button.
Press the Yes button to add the thermostat to the Almond+.
he orange light on the thermostat should blink while the thermostat is being added and then turn solid, while the LCD UI on the Almond+ should let you that it's adding a new sensor, then you should get a small box where you can change the name and location. Once you've tapped on done here, you should be able to see the thermostat in the Connected Sensors option in the LCD UI.