I've had the same problem, though not being as thing as you, but couldn't really gain weight either. Never really being hungry
I'm not in the mood for food in the morning, so only thing i really could eat was milkproducts, like yoghurt.
Turned to drink weightgainer instead as breakfast(with milk), biking to the place i should (7-15 km away). Biking fast obviously since i don't want to spend my entire day on the bike :P
Gains you some weight on the legs.
When i'm not being active i lose weight, because i eat too little, thought it feels allright.
Trying to eat varied food, with a good amount of meat in too, which i want when i'm active. Otherwise i can't be arsed to eat anything
Drinking weightgainer before i go to sleep.
That last part can be troublesome if you're prone to sugar-rushes, but it has stopped for me after a while.
Should maybe add that i went from 60 kg to 71 kg in a few months, but i was a bit extreme with the activity too :P