You'll need to add following Apache Commons library to the classpath: commons-httpclient.jar
(you would put it in the same folder where other JARs reside that are used by your nutch installation).
You can find the current version of HttpClient here http://hc.apache.org/httpcomponents-client-ga/
Please note that it is possible that your Nutch version uses an older version of the HttpClient and the current version of the HttpClient is not backward compatible with that older version. In this case you'll need to download that older version of the HttpClient and include that older version within your libs.