interestingly it works when you put all the parameters in the call to Create the instance of HttpWebRequest like this:
Code:
HttpWebRequest httpWebRequest = 
   (HttpWebRequest)WebRequest.Create("http://friendfeed-api.com/v2/entry?body=Hello API&format=xml");
and skips the line
Code:
Stream dataStream = httpWebRequest.GetRequestStream();
"Hallo API" was successfuly added to my feed