I can/t seem to gain weight, I have normal appetite, what can I do?

Talk to your doctor. You have an interesting problem that many obese people would love to have. Not to be kidding since this is serious, the first thing to do is to be thoroughly examined by a family doctor. Your thyroid gland needs to be examined also with blood tests. Then your diet history should be reviewed, studied, and tested to see if you are eating more calories than you are expending each day.
Eat more, call doc. Your inability to gain weight could be related to something as simple as a naturally fast metabolism and not enough calories, but there are serious medical conditions that can cause this problem. If you are eating three meals a day, not over-exercising, and still can't gain weight, you may need blood work and a physical exam to rule out a more serious cause.