Do like steve says. Some professional carpet layers I know now drill and use 3/4" concrete screws. It is more work and they charge an extra dollar a yard but it is easier than being called back and do it all over again when a nailed tack strip pulls loose from the concrete. It depends on the concrete but as steve says the nails set with one hit and another just knocks them loose. Some (and some don't) also squirt a little carpet adhesive in a big S pattern to give the pad a slight gluing down so it don't move but it is the same as stapling the pad on a wood floor, some do and some don't.