You will need to Add a Device to your account.
Access Device List
Click on the device name at the top of the homepage to access the Device Lookup ("Devices" menu).
Add a Device
Click the Plus (+) icon, at the top right of the Device Lookup menu.
Proceed to Device Activation
Select the generation of controller you have, below:
Generation 2 Rachio? See How do I activate my controller (Generation 2)
Generation 1 Rachio? See How do I BlinkUp my controller (Generation 1)
Follow the connection process(es) above, per your selected Generation.