use rom manager to easily install the latest 22.214.171.124 CWM (if i not mistaken) to flash the new rom. Your recovery is probably outdated. Otherwise you can flash the latest philz recovery from nexus s android development forum.
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?