Hrrm. Eat junk food while not walking? I lost about 25 lbs. when I quit eating Lunchables two years ago, maybe you need to try them. Stomach flu helps too. Los another 18 lbs. from throwing up anything I ate, eventually lying in bed, only drinking four glasses of water a day and no food for five days.
But in all seriousness, if you need to gain weight, I'd recommend eating a lot of junk food. Anything that burns good(What? Fat is really flammable.) makes a fatty food. Also, anything greasy.