If you wait until later, the sheet might no longer be there. Attend the experiment and have the experimenter sign your receipt. This signed receipt is just like the slip you get back from the clerk at a store. It is proof that you actually did the experiment. In the event of a mix-up, take the receipt to the HSP coordinator in order to receive the credit you earned.
Mistakes are rare, but they do happen.
Don't lose your receipt and make sure the experimenter signs it. Secondly, we ask that you not sign up for more experiments than is necessary to complete your requirement. We want to give as many students as possible the opportunity to be a subject. Your cooperation is appreciated. Failing to show up is defined as being more than 10 minutes late for your appointment without having first cleared it with the experimenter.