KCT_Den_File_Openner-1.9.1

This version supports recent ImageJ and Fiji and can be installed simply putting the KCT_Den_File_Opener-1.9.1.jar file into the plugins folder of Fiji or ImageJ.
In the File menu there will be new items "Open DEN ...", "Save DEN ..." and "Open DAT ..." you can also drag and drop *.den and *.dat files.
You can read more about the DEN format here

Den file opener v1.1.1

Install imagej and run it to create application directories.
Put the jar file KCT_Den_File_Opener-1.1.1.jar into .imagej/plugins directory.
In the File menu of your ImageJ you shall see Open DEN...