In v20 its available in the Folio Builder panel (iPad only, I think). You have to plug to your Mac to the iPad with the USB cable and starting the Adobe Content Viewer app on the iPad first. Otherwise check iTunes on your Mac if your iPad is listed under "Devices".
Are you using InDesign CS5.5 or InDesign CS6?
You have to be up to date with the Overlay Creator panel to v20 as well. Check for that.
Recently I had no chance with InDesign CS5 and the v20 tools installed on a Mac with OSX 10.5.8.
The v21 is supposed to have it like this
Peview on Device for active layout (iOS)
Before v21, Preview on Device was available only for entire folios. With v21, you can now use Preview on Device in the Folio Overlays panel to preview the current layout (iOS only). Connect your iPad or iPhone to your computer and open the Adobe Content Viewer app. In the Folio Overlays panel, choose Preview on [device name].
the device list in v21 are available in:
— the list of all your folios in the folio builder panel (not the article
— the overlay creator panel (CS6: "Interactive Overlays")
Europe, Middle East and Africa