during 'stolen hearts', you need to get the guard out of his booth by buying/stealing all the doughnuts in the cafeteria. once they're all in your possession, agree to sell one to the nurse. if you have her compromising photo and at least 2 nurse lust points, you can ask to start a tally instead of charging her money — the doughnut will add one strike to it, which can be used later as a trade for favors.
then, go to the nurse's office and click on the strike book on her desk to use your first strike!