You need to attach a Yellow Wonderful to your Hoe. Then charge it and use it to til the ground in the Rice Paddy. Then plant the Rice Seeds in the tilled spots. You'll need to attach a Yellow Wonderful to your Sickle when you want to chop the fully grown Rice. You don't need more then one Yellow Wonderful though. Just have Charlie switch it between your tools.
(Since it is Summer the Rice will probably not grow in time. And end up dieing by the time Winter comes around)