I hope this is close to what you are looking for. Some people make jelly from beachplums but I presume you are looking for a jam recipe.

Beachplum Jam

2 tablespoons lemon juice
4 cups plum pulp and juice*
1-1/2 packages of powdered pectin
7 cups sugar

*To make the plum pulp, cover several cups of plums with water and cook them slowly until mushy. Then put them thru a food mill to press out the pulp and juice, leaving behind the pits and skins. (Making the jam this way gives you a a thick and tangy jam -- different tasting than a jelly recipe.)

Put the lemon juice and beach plum pulp in a deep nonreactive pot. Bring to a boil. Add the pectin. Return to a boil. Add the sugar slowly, stirring constantly. Bring to a boil again and boil for 1 minute, or until the jam sheets off the spoon. Skim off any foam. Pour into hot, sterilized jars. Cover with new, clean, hot caps. Invert for 5 minutes (or use a boiling water bath).

Makes 7-8 8-oz. jars