I have a NPAPI plugin, incorporated into Chrome extension, and defined in manifest file as public. When an object tag with the plugin's mime type is inserted into background page of the extension, the plugin loads ok. When the same object tag is inserted into an arbitrary webpage using chrome.tabs.executeScript, it fails to load. If the plugin itself is placed into Plugins folder (and loaded into the browser), then the objects with the appropriate mime type are inserted into arbitrary web-pages successfully.
The question is why the plugin does not load properly into arbitrary web-page, when it is deployed within the extension and marked as public (so it is supposed to be available for any page, as far as I know).
Posting an answer with my comment(s) as requested =]
If you install an extension with a public plugin (and everything is installed correctly) then it will show up in about:plugins. Therefore, there is something wrong with your extension/plugin installation. unfortunately, that's all I know.
about:plugins as noted in the comments will display all plugins that are public whether installed as public plugins in an extension or just installed as NPAPI plugins.