If you intend to do it by the defaultroutine _mulliganRules.Add(new Tuple<string, string>("True", "card.Entity.Cost >= 0")); where 0 is the threshold mana cost. If you intend to do it with a mulligan in your data folder, there seems to be a bug in which you can't change the value for mulliganing (it is stuck on > 3).
If you don't know how to do it. Open your HB folder, inside the routine there should be a defaultroutine.cs which you can edit with the notepad (I recommend using notepad++ or your beloved IDE for easier edition). To edit the mulligan.txt, you should probably use HSBtoolbox which is a wonderfull tool that can help you to create it.