InDesign CC 2015 supports the DPS 2015 workflow -- that workflow does not support the Desktop Content Viewer. That's why there is no Preview at the bottom of the Overlays panel.
All content proofing is done through a preflight process that's started in the DPS 2015 Portal, you need a DPS account to sign in to this portal. Then you can preflight to a custom preflight app you build in the portal or use the DPS Preflight app.
HTH
