NGSPICE is released as "gzipped tarball". The source tree is archived with the tar command and the result compressed with gzip. Since the development of the NGSPICE is carried on UNIX (mainly), all the files have the "line feed" character as newline character and this may cause problems when working in non UNIX environments (like DOS). If you are going to extract NGSPICE on system that use different newline character, you will have to convert the files with some utility.
To extract the NGSPICE archive you need the tar archiver and gzip compression utility:
pnenzi@janus:~$ gzip -d ngspice-rework-15.tar.gz pnenzi@janus:~$ tar -xvf ngspice-rework-15.tar
Once extracted, you have to enter the source tree (just cd into the top level directory).