Step 1: Click the three dots and select the "Edit" button.
Step 2: Zoom in on the map using your mouse/trackpad or +/- buttons.
Select a billboard by clicking on one of the red dots and then click the "add sign" button. Once a sign is added the dot will turn green on the map.
Click "remove sign" for signs you have already selected that you would like to remove. Once a sign is removed the dot will turn red on the map.
Step 3: When you're done selecting billboards, click the "Save & Close Campaign" button.