There is a Default property on a control under the taskbased dialog you can set to true. From the sample (Samples\TaskDialogDemo\CS\TaskDialogDemo) that ships with it:
TaskDialog tdEnableDisable = new TaskDialog();
tdEnableDisable.Cancelable = true;
tdEnableDisable.Caption = "Enable/Disable Sample";
tdEnableDisable.InstructionText = "Click on the buttons to enable or disable the radiobutton.";
enableButton = new TaskDialogButton("enableButton", "Enable");
enableButton.Default = true;
enableButton.Click += new EventHandler(enableButton_Click);
If you run the demo, click Enable/Disable sample and then press Enter a few times you will see that the two buttons take turns being the default.