In the Masthead of the Journal Management pages, OJS has the option to either have people with certain roles automatically added to the masthead or to create a custom Masthead yourself. Reviewers is not one of the roles included in the automatic option. It is only Editors, Section Editors, Layout Editors, and Copy Editors. If you would like to include a list of the Reviewers in the Masthead, you should deselect the automatic option and add people you wish to have listed, either on the existing page for Editorial Team or on a new page with whatever title you wish (this title will show up in the About pages as a link under Editorial Team).
To learn more about setting up the masthead, there is an OJS tutorial here: http://pkp.sfu.ca/tutorials