I was trying to create an example plugin based on the documentation at http://pkp.sfu.ca/ojs/docs/technicalreference/2.1/pluginsSamplePlugin.html

My plugin appears in the list of plugins, and I have enabled it. The plugin appears to have no effect, though. The list of Management pages on the Journal Management page does not have the additional item the plugin is supposed to generate.

I spent some time debugging this and discovered:
the ExamplePlugin.inc.php file includes this line
import('classes.plugins.GenericPlugin');
but there is no GenericPlugin.inc.php file in the classes/plugins folder. Is there supposed to be a class file there or is the example code and/or documentation wrong?

the DataversePlugin.inc.php file includes this line import('lib.pkp.classes.plugins.GenericPlugin');
the file lib/pkp/classes/plugins/GenericPlugin.inc.php does exist.
I commented out the original import line from my ExamplePlugin.inc.php file and copied the one from the Dataverse plugin, then disabled the plugin and re-enabled it. I still see nothing rather than the expected output from the plugin.

By the way, I've set display_errors to E_ALL and am not seeing any php errors.