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