private void PopulateResultNavigator(IQueryResult qr)
{                       

    if (qr.Navigators() != null && qr.NavigatorCount() > 0)
    {
    IEnumerator nav = ((System.Collections.ICollection)qr.Navigators()).GetEnumerator();               

    while (nav.MoveNext())
    {
        Com.FastSearch.Esp.Search.Result.INavigator navigator = (Com.FastSearch.Esp.Search.Result.INavigator)nav.Current;


       //Get following here
       //navigator.Name
       //
       //navigator.Type.ToString();
       //navigator.ModifierCount();

        System.Collections.ICollection modifiers = null;
        try
        {
        modifiers = navigator.Modifiers();
        }
        catch (Exception)
        { }
        if (modifiers != null && modifiers.Count > 0)
        {
        IEnumerator mod = modifiers.GetEnumerator();

        while (mod.MoveNext())
        {                           
            IModifier modifier = (IModifier)mod.Current;

            //Get Following here
            //modifier.Name;
            //modifier.Attribute
            //modifier.Value
            }

        }
        }                   
    }               
    }
}

Com.FastSearch.Esp.Search.Navigation.FilterTerm term = new Com.FastSearch.Esp.Search.Navigation.FilterTerm(string Name, string Val, bool plus)
//put plus as true so that your drill down parameter gets added
//As user drills down, keep on adding terms with space seperator in a different variable say filterterm like below

filterterms = filterterms + " " + term.AsString(true);
//Now add this filterterm to navigation search parameter
query.SetParameter(new SearchParameter("navigation", filterterms));