1. Join a calorie counting website. I use fitbit.com because I had their pedometer, but there are others as well. You don't have to calorie count all the time, but on the days when you do so it's like taking a snapshot of your daily calorie intake… you may discover you eat less than you think you do. Then you can change your meals, and find out if they have the calorie content you need for your daily intake… when they do, just keep eating similar meals and you don't have to calorie count all the time.
2. Add healthy fats such as olive oil and coconut oil (refined has a neutral taste) to food in abundance. Especially starchy veggies are actually not calorie dense, so add on the fat.
3. Eat a lot of calorie dense food: legumes, lean meat, diverse grains. Top with healthy fat.
4. Eat a lot of avocados.
5. Eat a lot of nuts and seeds--raw or toasted (not roasted) to get the maximum nutritional value out of them. Calorie-wise and nutritionally they're great, but be careful because they make you feel full quickly--this is why people can eat them while trying to lose weight. To counteract this, eat them with something sweet, since the sweetness is an appetite stimulant: add dried fruit or chocolate chips to nut/seed mixes, have them in nut butter form on fruit slices, or add a lot of nuts/seeds to smoothies (green smoothies while you're at it). Also, if you're having smoothies, be generous with bananas and again avocado.
6. For dessert type dishes, have banana or other fruit, and top with something heavy in healthy fats and moderately sweet… I think of these as somewhere between a banana split and a fondue.
7. Dairy turns out to not be the best thing to eat/drink regularly, but it's great now and then. Full fat yogurt, or low fat yogurt with some healthy vegetable fats mixed in (as in tsatsiki, I think)… ricotta cheeses, in savory or dessert dishes… hard cheeses on crackers… really whatever you like.
8. If you would eat more if your food tasted better, concentrate on herbs, spices, and all those sorts of flavorings, especially in your fats. Also acids (but not too much or it suppresses appetite). Bland food is the secret to why all weight loss diets can work.
9. Minimize the liquid in your meals, unless it gets you to eat more overall. Have stews instead of soups, for instance.
10. This won't help you gain weight, but eat a variety of greens and veggies to keep up your general health. The easiest way is to do this in (high calorie, adequately sweet) green smoothies, so if greens don't happen in other meals at least they happened there.
The first thing I would do is to see your doctor to make sure nothing metabolic (thyroid issues, or poor digestion for example) is keeping you thin. In those cases, sometimes even if you're taking in the recommended number of calories per day, your body can't absorb them. Good luck!