Hi.
Although you need to add more calories to your diet to gain the needed weight, the key here is to add healthy calories, not junk food. Since you are an active person, a diet of protein 30%, carbohydrate 50%, and fat 20% might be sufficient.
For breakfast, you can add whole-grain bread and peanut butter. For afternoon and evening meals, you can add bigger portions of baked potatoes, baked chicken and fish.
Make your meals balanced. Always have a healthy protein source such as low fat meat, legumes, chicken, or fish and add green vegetables and potatoes. Beverages should be milk or fruit juices, not sodas.
You would still need to exercise. The proper regimen would be that of resistive training exercises such as weightlifting, like what you are already doing. Avoid running or bicycling as these aerobic exercises may cause you to lose weight instead of gain it.
Hope this helps.
Carbs Carbs Carbs less cardio for now. Low reps high weight. don't forget to stretch or you will lose flexability. A lot of people think you have to eat tons of protein, all that does is make you have gas and is hard on your kidneys. Your body uses carbohydrates as its first energy source. Still eat your protein though. It sucks but you may have to cut out some of your cardio or you'll be using all your energy for that. Its all about nutrition and hard work. I gained 65 on my bench and over 100 on my squats just because I started eating right. let me know if it helps.