We don't currently support previewing to an attached device for DPS 2015. The only way to preview in DPS 2015 right now is to build your own preview app in App Builder. We realize this isn't the most efficient workflow and are working on ways to make it better.
Only the new Digitial Publishing Solution is supported under InDesign CC2015. To use the old feature in DPS Classic, you'll need to use InDesign CC2014.
We don't currently support previewing to an attached device for DPS 2015. The only way to preview in DPS 2015 right now is to build your own preview app in App Builder. We realize this isn't the most efficient workflow and are working on ways to make it better.