You don't mention what url your code actually calls, but I guess that your method is parsed as a file/path, and not a parameter.
var client = new RestClient("http:// mysite.com/api/");
var request = new RestRequest("v2.php", Method.GET);
request.AddParameter("method", "information::method");
request.AddParameter("token", authenticationToken);
request.AddParameter("target_id", targetId);