Here is a code sample that will show you how easy it is to save a model:

var parser = new CsvParser(() =>new StringReader(Resources.AptitudeData));
var observations = parser.EnumerateRows(v => v != "Pass").ToF64Matrix();
var targets = parser.EnumerateRows("Pass").ToF64Vector();
var learner = new ClassificationDecisionTreeLearner(2);
var sut = learner.Learn(observations, targets);
var writer = new StringWriter();
sut.Save(() => writer);