How to add Destinations: 

1) Under "Products" menu on left side of Builder 

2) Select "Price sets" 

3) Click "Destinations" 

4) Click "+" icon at the bottom of builder 

5) Change "Destination Sets" to the country you wish to add or leave as custom if not listed, If custom is selected you will need to add "Country" and "State"/ province 

Make sure to preview and test before publishing.