How Open Map and Save Map buttons work:
When you click the "Open Map" button, K-2D will open a dialog box asking you the location and name of the map file you want to open. If you have never changed the default directory and file extension, then you'll see the contents of the folder "Maps" that lies under K-2D installation folder. This folder will show you all ".map" files you have created till that moment.
If you have changed the default directory (by pointing another folder when opening/saving a map file) then you'll see the contents of the last folder you have used. At this point, you can point either: a ".map" file or a ".dat" file. K-2D will automatically show you the last file type you have used for a open/save operation.
If you point a regular ".map" file, then K-2D will capture information about the data-file and tile-set-file from it. Once a ".map" file is open, whenever you click the "Save Map" button, K-2D will automatically save the map data to the correct data-file, and update the project file with the name of the current tile set (if it has been changed).
In the other hand, if you have pointed a ".dat" file when opening a map, then you are acessing the data-file directly. When you hit the "Save Map" button to save the file, K-2D will save the data to the correct data-file that you have opened, and will create a project file for the current map at the default Maps folder under K-2D installation directory.
When you save a map file, you also may choose from 2 file extensions: ".map" and ".dat". If you save a map with the ".map" extension, then K-2D will save the project file (.map) and the data-file (.dat) at the folder you point. If, in the other hand, you save the map with the ".dat" extension, then K-2D will save the data-file (.dat) in the folder you pointet, but the project file (.map) will be saved in the default Maps folder under K-2D installation directory.