I don't know whether or not the voice actor doesn't want credit. The problem with voice actors is it's hard to even get a clue about age or gender. For all I know the voice actor could have died between the time Inside-Out Boy stopped making new stories and imdb.com launched.
Sometimes for old media finding references is difficult. I had the same problem trying to credit an actor when I reviewed the only public domain episode of the 1950s Adventures of Superman TV series.
RE: Nickelodeon's Worst Superhero