If you cannot find the freeze plug, it is probably because you have no idea what you are looking for. First off, there isn't just "A" freeze plug on an engine, there are numerous ones located all around the block and sometimes on the cylinder heads as well. Some are concealed between the engine rear and firewall, some may be concealed behind accessories such as the power steering pump, water pump, or alternator, and yet others may be hidden between the engine and transmission. A freeze plug is a recessed metal disc that is pressed into the casting to allow expansion of freezing water to pop it out, preventing the casting from cracking in the event of a freeze-up in extremely cold conditions. Unfortunately, these plugs corrode over time and may leak, requiring replacement. The rule of thumb is that if the offending plug is accessible, replace the bad one and move on. If it is not accessible and you need to remove the engine to provide service, then you are best suited to replace ALL freeze plugs since you already have access to them and if one has gone bad, the others may well soon follow. You wouldn't want to go to the expense of pulling the engine to replace a rear freeze plug, only to have to it again in the short-term future. Here is a shot of some freeze plug openings....one with a plug in it and two that are open. Additionally, removal of the freeze plugs also allow for more thorough cleaning of the engine block in the chemical tank during engine overhaul and rebuilding processes.