The way to train a Pacman to eat from tongs is simple, but depending on the particular frog's personality determines whether the frog will accept the food or not.
Visibly show the frog that there is food on the tongs, but do it at a distance of about 4" to 5" above the frog. Drop the food item from the tongs so that it lands about an inch to an inch and half away from the frog. Instinctualy the frog will strike at the food once it lands without thinking. Doing this over time will get the frog to realize tongs = food and the frog will eventually just take the food from the tongs.
You can also use the methods from Jim and Jessica Ivory of Ivory Reptiles.
Tong Feeding 102 - YouTube