I followed the tutorial of Phonegap from the official Phonegap.com site (getting started with Android).
I have created project with following all the steps.
Had same problem with Class not found. One problem to look at is to ensure that the android.json (or ios.json) file is being updated on a build. Mine was effectively empty. Also, remove and add back the plugins as mentioned in some other posts. Finally, the thing that worked for me was to ensure that the plugins were corrected referenced in the config.xml:
Note the double naming of 'device.Device' and also the sub-classing of the 'camera' and 'dialogs' plugins. This just isn't properly referenced by the Phonegap documentation.