You may have clicked on a separate "Apply Now" or "Click to Apply" button within the job description that takes you to a site outside of Monster+HotJobs. If this is the case, the employer has chosen for all interested candidates to apply using their own applicant tracking system, which does not recognize your Yahoo! username and password.
Please follow the instructions to apply to that job as listed on the screen. You may need to create a separate username and password for that site.