My nipple is slightly cracked and itchy. What should I do?

The situation as you describe it is not worrisome in my opinion. I recommend that you first moisturize the cracked skin with an unscented moisturizer (e.g. Glaxal Base). Moisturizing should help heal the damaged skin, as well as significantly reduce the itching. If the itching persists after about 2 weeks of application, or if signs of infection appear (enlarging redness, heat, induration, etc.), I advise you to talk to your pharmacist, who can recommend another cream.