Inspect the black rubber cover that fits in the disposal. Sometimes bits of garbage collect under the flaps and create a smell.
Clean the rubber cover, if necessary. If yours lifts out, remove it and clean away debris with a scrub brush and warm, soapy water. If your rubber cover is installed permanently, lift up each flap and clean it with soapy water and an old toothbrush.
Deodorize the disposal. Cut a lemon in half and drop the fruit and a handful of baking soda into the disposal. Turn on the cold water faucet and then the disposal. The unit will clean itself as it grinds up the mixture.