He got bounced after, like, the first show. He can't even walk, I'm not sure what made him think he could dance (except for the free pub).
He actually didn't get bounced after the first show... he got the lowest score of that episode, but remained in the competition due to viewer votes. Woz later posted online that he felt that the vote count was not legit and suggested that the DwtS judges had lied to keep him on. Then he was briefed on the method of judging and vote counting, and retracted what he said. He went on to pull his hamstring and fracture his foot, continued to compete, and was finally eliminated on the March 31 episode.
Regarding what made him think he could dance, I'm guessing his previous relationship with Kathy Griffin in 2007, and his appearances on "Kathy Griffin: My Life on the D-List" had whetted his appetite for being on TV... after all, DwtS isn't about people who can dance, it's about people who are kinda famous wanting to be on TV, really. If you want to watch a real dancing show with actual talent, I'd recommend So You Think You Can Dance?