As a question, a check box type has 2 possible answers; unchecked (clear) says one thing and checked (selected) says another. You can't require registrants to select the check box because leaving it blank is actually one of the possible answers.
Consider the question "Will your child be participating in the post show activity?". With it being a check box type, selecting the check box says "Yes, my child will be participating." and leaving the check box clear says "No, my child will not be participating.". Both are feasible answers. If you were to be able to make the question required, you would be forcing registrants to say that their child will be participating. The better question type to use for this example is a Yes-No which you can make required.
If you are asking the registrant to agree to something, such as your policies/waiver, use the Agreements tab. Each agreement text record is followed by its own I Have Read and Agree check box on the Event Registration Form. The customer must check these boxes and enter their e-signature in order to submit the registration form.