google docs in asp.net

Question

GoogleSpreadsheetUtility gs = new GoogleSpreadsheetUtility(txtUsername.Text, txtPassword.Text, ddlSpreadsheets.SelectedValue);

ERORR:
The type or namespace name 'GoogleSpreadsheetUtility' could not be found (are you missing a using directive or an assembly reference?)

I am trying to get reference but i couldnt, can u please give path/solution.