Initially, the content of the following dropdowns: towns, localities, and post towns, is pre-populated by our system from the source data. In order to add the missing entries please go to Settings and on the Gazetteer Defaults tab click the Localities and Towns Edit... button.
A new window will pop up with some values already populated from the source data. Please add any missing values there and click OK when you're done.
Please reload the GMS app (press F5 to reload the web page) and now, when you open New Address pane, you will see the formerly missing localities or towns on the lists.