The map in D-RATS is automatically downloaded on an as-needed basis from openstreetmap.org.  As a result, you must be connected to the Internet to be able to download maps.  All that is required for this is setting your Latitude and Longitude in Preferences and then opening the map window.  Maps will be downloaded automatically and will begin to populate the window in a few seconds.

Once you have downloaded map information for a particular location at a particular zoom level, the maps are stored locally and do not need to be re-fetched from the Internet.  Thus, you can be out in the field without Internet access and still see any maps you have previously downloaded.

D-RATS includes a tool called the Map Downloader which you may use to download a large region of maps automatically so that they are cached on your system for later offline use.