Marc can be found hanging on the fence on the second floor of Palisades Mall near Ned's Knicknackery (Dead Rising 2). He was trying to escape from zombies jumping from second floor but instead he caught himself trapped, unable to climb back because of zombies waiting him there or jump down because it's too high. When Frank finds him, Marc will tell Frank that he can't climb up and he needs a stuffed toy placed under him to land on it safely.
Unlike other survivors you don't need to talk with him to convince him to follow you, he will join Frank immeadetely after safe landing.
You can use any stuffed toy to save Marc: Rabbit, Donkey, Elephant, Bull or Bear. The toy becomes unobtainable after Marc's landing.
The nearest place where you can find a toy is Ultimate Playhouse on the 1st floor with Giant Stuffed Bull inside it.
If you fail to find any toy in time Marc will fall down to his death and the nearest zombies will devour his body, adding some brutality to his dramatic fate.
This mission might be a reference to the Hanging by a Thread mission from Dead Rising where Frank finds two survivors Nick Evans and Sally Mills hanging on a giant inflateable rabbit trying to escape from zombies.