You are selecting the right usertype. If you want to login as a reviewer make sure you have selected Reviewer from the drop down box.
You are not already logged in as a different usertype. ie. If you are logged in as a Reviewer and then go to login as an author, you MUST logout first.
You cannot login to reviewer and author pages with one login. You must register one login for each usertype.
Send an email to firstname.lastname@example.org if you are still experiencing problems.