To display questions to your class without revealing the answer, you want to use the Live View page from the Plickers website. The question view from the Library or your mobile device is intended for the teacher's eyes and thus has the answer revealed. When you display the question on the Live View page, it hides the answer by default, but gives you the option to reveal the results in aggregate, or by individual student.
For full instructions on how to use Live View, see our detailed overview.
For more information on how to configure the Live View page, you can see that here.