If you are giving an exam on Moodle that is open to take from anywhere, and students are unable to access the exam, you may want to first troubleshoot these two steps.
moodle
Emailing your Class in Moodle
Moodle currently does not have an internal email function associated with your course.
To send an email to your entire class, you will have to use your University of Lethbridge email account.
Setting your Course to Visible
By default, all Moodle courses are hidden to students until you as the instructor are ready to make the course visible.