Nsa36,
the viewer and builder are essentially the same application. Go to File, Import Chemical System (e.g. xyz) or Import Calculation from Output File (e.g. ecce.out)
Note that you will only be able to open ecce.out files produced using the ecce_print directive -- you won't be able to open a simple piped output from an nwchem calculation (e.g. nwchem nw.in |tee nw.out won't yield a useful file)