First, you need to switch to Map View by clicking the Map View button on the toolbar above the Search Results:
On the map view, you'll see a column of buttons on the right-hand side:
The button with the Map Pin icon that is highlighted in the example image will bring up the Zip Code map (you have to be zoomed in to a certain degree for the zip codes to display, so if there are issues, zoom in on the map a bit more)
Now, when you click on each zip code, you'll see that the area for that zip code is highlighted:
And in the Search Window, the Zip Code field has each selected zip code:
Just hit the Search button once you've selected your zip codes and the results will then display.