From within your Firewall Overview, you will see a list of all your Servers and any Services (OpenStack or DRAC's etc). Locate the row for the Server or Service you want to Apply a Rule Group too and press the "Choose" button;
You will then see an option for a Drop Down list, this will contain all of the available Rule Groups for you to select from;
Select the Rule Group that you would like from the list and click the "Save" button;
That is it! The Rule Group is then applied to the server.
Alternatively, you can press "Cancel" at any time and the changes will be reverted.