Plugin instructions are here:
I'm not familiar with the post you're following on "Alf's blog", but for your reference:
How do I install a plugin?
- Locate a plugin of interest using the Index of plugins sticky thread.
- Upgrade to the latest Calibre version. Or at least to the version if specified on the plugin thread.
- Download the plugin zip file from the forum thread. Do not extract the zip file.
- In Calibre choose Preferences->Plugins. Click the Add new plugin button to browse to your xxx_plugin.zip file and click OK. The plugin should now appear in the list above. Apply and close.
- Restart Calibre after installing a plugin. Ctrl+R is a convenient shortcut for this.
- Your plugin can often be customized by selecting it within the list in the Preferences->Plugins dialog, and clicking the "Customize Plugin" button.
- By default a GUI action plugin will not appear in any menus or toolbars. To add it to any of these you must go to Preferences->Customise the toolbar. In the dropdown at the top choose where you want the new "action", such as "The context menu for the books in the Calibre library". Select the new plugin within the "Available actions", click the add button and move up/down as required.
If you're doing something other than just installing a plugin, perhaps you needed to extract something,