I tried didifferent ways to add some kg's but no matter what i do it doesn't work out... do you what might be wrong with me...

Adding Weight: Weight is somewhat predestined genetically. If both parents are large then the chances are the kids are going to be large without diet and exercise intervention. Protein is the building block of muscle. Try and eat 2-3 gram of protein per pound. At the same time you are going to need to exercise to build muscle mass with strength training. This should add weight. Cut down on carbs.