Apart from a constitution which easily dispenses with calories, it's essential for this Somatotype to obtain a higher-than-average quantity of energy (food) to support a mass gaining program (if that’s the intention).
Therefore, due regard should be paid to getting in sufficient amounts of complex carbohydrates to bolster weight-gains. Although the diet (I hate that word) should be higher in calories – as mentioned above, it doesn't mean open season on junk food every day. Enjoy those 'pleasure foods' by all means – but ensure adequate intake of nutrition-dense foods.
Meal-frequency can be instrumental for 'hardgainers.' Meals are best not skipped, especially if you've traditionally eaten the usual 'three meals a day', and want to add mass. Again, you should look to increase daily consumption of good quality food – and more often.
Remember, these are guidelines. It almost always comes down to you: the individual, and to try things – adjusting as necessary.
An additional point to consider, particularly if you're struggling to gain weight, is to experiment with consuming an additional small meal in the late evening. This will help avoid undue catabolism (muscle-breakdown) – especially if you're following a regular training protocol.