Hello and welcome to the Community!
Displays are added from the Player and not the CMS (hence no Add button). On installing the Player you will be asked to provide the CMS Address and Key which will make the connection to the CMS. Once connected Displays will then be shown in the CMS.
Select your chosen Player from the Player Installation guides to help you.
The CMS key can be found from the Settings menu under the Administration section of the CMS menu.
On another note, v1.7.9 is a very old version, we are currently on v3.0.3, so you may want to think about an upgrade to a newer version so that you benefit from bug fixes and security patches:
Any further questions then please do come back to us.