Instructions
Point your Wii remote at the screen. The menu bar will appear at the bottom of the screen. Place the cursor on the first icon in the menu, the suitcase, and press "A."
Place the cursor on the fishing rod in your pocket, press "A" and choose "Carry" from the submenu that appears. Your character will hold the fishing rod.
Look in the river for keys, which will cast mid-sized shadows that look like black fish.
Press "A" while facing the river to cast your fishing line near your targeted shadow. Wait for the shadow to take your line down (all shadows nibble the line a bit before taking it down). Press "A" again or pull back on the Wii remote to pull the key out of the water.
Take the key to the neighbor who lost it. He will put a reward in your pocket to express thanks.