I\'m trying to set up a dictionary for my MKMapItem, to open the native Map Application set to a certain location with a certain directions mode (walking, driving,
You are looking for the MKDirectionsMode Enum.
launchOptions.Add(MKDirectionsMode.Driving, LaunchOptionsDirectionsMode);
This is found in the MapKit namespace.
UPDATE
You don't need to use a dictionary, in your case you can use the MKLaunchOptions class to set the directions.
var launchOptions = new MKLaunchOptions();
launchOptions.DirectionsMode = LaunchOptionsDirectionsMode;
endingItem.OpenInMaps(launchOptions);