Combine plums and water in 2 1/2 quart saucepan. Heat to simmering. Cook for 3 minutes. Add sugar. Simmer about 1 hour until candy thermometer registers 220ºF. Add walnuts and rum. Pour into 5 sterilized 1/2 pint canning jars. Top with lids and rings. Process in boiling water bath for 20 minutes. Remove. Check lids for proper sealing. Lids should snap into concave position, indicating a vacuum seal. If jar does not seal, reprocess or refrigerate conserve until it is used.