Do you have your assessment set as a scored assessment or self assessment? (You can choose this option at the top of the assessment screen). The Show Me button can only be used in self assessments. If you have your assessment set as a scored assessment the Show Me button is disabled and will not work because this interactive is set to be used as a test of the students knowledge, therefore can be scored or graded. A self assessment is meant to be used to add interactivity to the course to make it more engaging to the learner and serves only to "self test" the learner to ensure they have learned the material covered. In this situation the Show Me button is present to assist the learner.
I hope this helps. Please let me know if you have any other questions.
Maria