Whilst DEM files share the same .ASC extenstion as a CivilCAD Ascii file, a DEM file can be recognised by the file header;
ncols 2000 nrows 2000 xllcorner 510000.000 yllcorner 6400000.000 cellsize 1.000 nodata_value -9999
Following the file header is the DEM data;
To read in a DEM file, go to: File => Data Input => DEM => DEM
