There is a lot of examples in codeplex. Did you read it ?
Well, here a sample from the Winform demo :
SummarizerArguments sumargs = new SummarizerArguments
{
DictionaryLanguage = "en",
DisplayLines = sentCount,
DisplayPercent = 0,
InputFile = "",
InputString = OriginalTextBox.Text // here your text
};
SummarizedDocument doc = Summarizer.Summarize(sumargs);
string summary = string.Join("\r\n\r\n", doc.Sentences.ToArray());
// do some stuff with summary. It is your result.