As one of the comments mentions, you can bypass this whole step by using Reaver,...

phusion on Aug 28, 2012 | parent | context | favorite | on: How I cracked my neighbor's WiFi password without ...

As one of the comments mentions, you can bypass this whole step by using Reaver, which attacks the WPS pin number instead of the encryption scheme. It's not 100% and it takes 8-12 hours to complete, but it does work.