Click Find source:

You can now search for sources by typing, for instance, a placename and clicking search: 

All avaliable sources in the Digital Archives with a title corresponding to your search will be listed in your hit list:

You can also specify the period, type of source or geography as well as or instead of the location. Please note as well that you can search by more than just the location. For instance will "kjøpstad" ("township") return all sources with this word in the title.