The ZIP file is an update of a package that has been around a while, but is not in the Knowledge Base. It includes two-y-axis Plot and ListPlot.
Un-zip the attached file and place the folder into one of the directories in the $Path variable in Mathematica. The two most common locations are the Applications/ folders under the locations given by
$BaseDirectory or $UserBaseDirectory
in Mathematica. (See http://reference.wolfram.com/mathematica/tutorial/ConfigurationFiles.html.)
After starting Mathematica, searching the Documentation Center for TwoAxisListPlot pulls up the guide page. The documentation for TwoAxisPlot could be more complete, but the link works.
Attachment: TwoAxisListOrLinePlot.zip, URL: http://reference.wolfram.com/mathematica/tutorial/ConfigurationFiles.html,