If A says something and you don't understand what A said: "I don't understand." If YOU say or do something and A (or other people) don't understand what YOU said, then: "I haven't been understood." Also, if you say or do something and people think you meant something else or did it for a reason that is not the real reason: "I have been misunderstood." "Picasso's art was misunderstood by many people."

A pergunta deveria ser "Which IS the correct form to write?" Se você não entender algo, pode dizer o seguinte: "I didn't understand." ou, mais naturalmente "I don't understand." "I haven't been understood." significa que você não me entendeu direito.

they are both correct but mean different things