I borrowed a Loos Model A tension gauge, but can't figure out how to use it. Any advice?

Put the cable that you are testing between the two posts and the clip it into place on the other end. That will put tension on the cable in the small space between the two posts and force the needle across the guage.
http://www.ekomarine.com/l/Loos_Company/

At the bottom of the web page I posted it shows a pic of the tool in use.