Add agar flakes to juice, water, and salt, bring to a boil. (NOTE: If using agar bars, first break bars into small pieces first, soak in water for 30 minutes, and wring out excess water before adding to juice and water.) If a creamier texture is desired, dilute the kuzu in a small amount of water and add to the boiling mixture. Reduce heat and simmer 10 minutes. Add fresh fruit and simmer an additional 3 minutes. Pour into heat-proof mold or glass dish and let cool to room temperature. Refrigerate 2 hours or until firm.