For the HTML5 viewer, the page-turning effects are Sliding and Page per page. So if you want to have the flipping option for your issue you need to enable the Flash viewer first. The following steps will help you do that: 

1. From the My Publications page open the publication folder you wish and click the Edit Settings button of the issue.

2. Go to the VIEWER tab and check the Enable Flash Viewer checkbox.

3. From the Viewer Mode dropdown menu, select the Flipping option. 

4. Click the Save and Close button when you're done.