In the Agency Locations section you will see:

In the Agency Locations section you will see all the locations where you have set up the GHL promo engine. If you want to add a location you can simply click on the green 'Add Location' button. To be able to add locations in your agency account, you have to enter the agency API key in the settings tab next to location.

Upon clicking on Add Location you will get this window 

Here you can select your location from the dropdown menu and click on add location

After the location is added it will show up on your Agency Location section

You can go into a location directly from your agency dashboard by clicking on the blue 'arrow' button and it will open up the dashboard of that specific location where you can edit the promotions for it.