I feel the guys here should be a little more detailed when they say eat 6 times a day. Basically when they say eat 6 times a day it does not mean eat 6 mixed rice dishes a day coz that's just plain OP.
I am 5'9 and weigh a little over 50kg. Based on my lifestyle (office work and futsal/football once a week), I would need to consume 2,400 ~ 3,000 calories a day to gain weight. Seems daunting, but it's doable, though it needs a lot of training (for your tummy that is).
The tip is to eat small meals in between your main meals. If you are not paying attention to the nutritional labels on your food, is high time you do. It keeps track of your daily intake to ensure you hit your goals. For instance:
Breakfast = 4 eggs + 1 tomato = 350 kcal
Snack = 2 slices wholemeal bread + peanut butter = 350 kcal
Lunch = 500 kcal (mixed rice, chicken rice, etc...average number, could be higher)
Snack = 2 slices wholemeal bread + peanut butter + yogurt = 500 kcal
Dinner = 500 kcal
Supper = Mass Gainer + Milk = 850 kcal
Total = 3,050 kcal