There is a chance that the class may not have met yet or simply a matter of timing between your access and the teacher's update. The other option is that the teacher has opened the grade book and entered an assignment, but has yet to record any scores for that assignment.
ParentPortal uses "cookies" and this means your internet security is set too high, probably to the highest level of security, which blocks cookies. Depending on your browser, you will need to find the Internet Security area and turn it down a level or two so cookies are accepted.