Вы пробовали это:
range.Find.Execute(
ref missing, ref missing, ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing, ref missing, ref missing);
while (range.Find.Found)
{
//Get selected index.
// Do as you please with range...
//Positions: range.Start... range.End
//search again
range.Find.Execute(
ref missing, ref missing, ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing, ref missing, ref missing);
}