public bool MultiSelect {get; set;}

OpenItemDialog addToProjectDialog = new OpenItemDialog();
addToProjectDialog.Title = "Add To Project";
addToProjectDialog.InitialLocation = @"C:\Data\NewYork\Counties\Maps";
addToProjectDialog.MultiSelect = true;
addToProjectDialog.Filter = ItemFilters.Composite_Maps_Import;

bool? ok = addToMapDialog.ShowDialog();

if (ok == true)
{
  IEnumerable<Item> selectedItems = addToMapDialog.Items;
  foreach (Item selectedItem in selectedItems)
    MapFactory.Instance.CreateMapFromItem(selectedItem);
}