Tonto,
There are two mechanical methods for securing the rudder in the down position. The first is the clamping plates at the pivot. You should have a handle on one side that allows you to tighten down these. Older boats may have worn the plastic spacers inside the clamping plates so the rudder slips.
The other is the downhaul line. It is not recommended to cleat this if you sail anywhere that the rudder might be struck. Many have replaced the original cleat with a break-away cleat that pops open with excessive pull, protecting the rudder. Look for this line coming out of the rudder shaft. The other end is tied to the top of the rudder to allow you to pull it up.
Pulling up on the downhaul line is the usual way to get the rudder down. If you push the rudder down with your foot while pulling up on the line, it should go down easily. (Don't fall overboard!)
