Appeal to Doubtful Authority is when one uses ideas or concepts of an inexperienced person to support one's argument. One would use this authority figure or non-expert as evidence to enhance there authority over the audience. It is essentially a random persons opinion that is being accepted as a professional's opinion.
A real world example of this is Jenny McCarthy, an Actress, who became an anti-vaccine activist once her son was diagnosed with autism. She speaks out against vaccines which has caused many parents to listen and not give their children vaccinations. Jenny McCarthy is not a doctor, but by being a celebrity it has only further promoted her cause and statues as an "Authority Figure". That is why Jenny McCarthy is clearly showing appeal to doubtful authority.
An example of appeal to doubtful authority in a commercial, is Justin Timberlake and Peyton Manning promoting Sony cameras. This is an example of doubtful authority because Sony is using these big celebrities to entice buyers, since they will believe if I get a Sony camera then I will be just like them. Sony is also pointing out that these celebrities know which camera brand is better because they see them everyday with paparazzi. (Start video at 3.00)
Fallacy Examples
Finally, here is a visual example of appeal to doubtful authority...
