Unfortunately the Preview mode for modules, built with the Module builder, is not yet supported. This feature will be released in one of our future versions, but for now you cannot Preview content items of Module builder modules.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items