Brian Steinberg Senior TV EditorAn unlikely guest turned up amid the dozens of Broadway actors, “Today” hosts and other celebrities who regularly take part in NBC’s annual broadcast of “Macy’s Thanksgiving Day Parade.”In a two-minute ad from Comcast, actor Steve Carell took a turn at playing Santa Claus, part of an emerging tradition that has the Philadelphia cable-and-entertainment giant hoping to capture attention during the annual event, telecast on the network it owns.