First you'll want to find the file. If your logged in as a regular user most likely it downloaded by default into your user's /home directory. If not sure use the command find
to find it, something like this:
find / -name netscape-i686-pc-linux-gnu-installer.tar.gz
Once found, just cd into that directory where it resides, then check out our tar glossary term here
or man tar
for all the details in unzipping a tar file.