Instructions for prj.adf  (used with .flt files)
1a) Works best with a coverage .flt file.
1b) Import the .flt using the float to grid toolbox tool
1c) copy prj.adf to the grid directory

Instructions for ndfd.prj  (used with .shp files)
2a) Use Arctool box : Define projection wizard
2b) Select file (.shp file)
2c) Select coord system -> select -> browse for NDFD.prj