Macbook down arrow key not working.

You could try a can of compressed air used for computers. Perhaps some dust or a crumb has got under the key. Follow the directions on the can.

If that doesn't work take your MacBook into an Apple Store or an Apple Authorized Service Provider as it may need more service than a simple can of air.

Edit: Before you do that restart your Mac. Sometimes that will clear any glitches that may arise.