I would try using 3-5% by weight of flour. How you use the dry milk will depend on what form you have. KA has two dried milk powders, Baker's Special Dry Milk and Dried Whole Milk (on pages 44 and 45, respectively, in the latest The Baker's Catalogue). The Baker's Special Dry Milk is a low-fat baker's grade dry milk that has been heated at high temperature and otherwise processed to disable either the whey protein or an enzyme (KA once told me that it is the protease enzyme) so that it doesn't result in an overly slack and soft dough. The dried Whole Milk is not similarly processed. Consequently, the best way to use it is to reconstitute it in water, heat it to the scald point, and let it cool off before using in your formulation.
Either way, the dry milk should result in a softer, more tender crumb. It was once described to me by a KA employee (I was told she was a baker) as having a "Wonder" bread texture. The dry milk will also increase browning of the crust. Dry milk has about 50% lactose, a form of sugar that is not metabolized by yeast, so it remains in the dough to the end to be available as a residual sugar to promote browning. It will also have a low sweetness value and not contribute to sweetness in the crust. A lot of bakers will use dried dairy whey instead of dry milk powder, mainly because the whey has a higher percent lactose (70%) but, more importantly, it is quite a bit cheaper than using dry milk. Of course, you will also get some calcium and other minerals in the dough from using the dry milk.
If you decide to go forward with the use of the dry milk, please let us know what kind of results you get.