click apply after customizing., shut down and reboot...sometimes that does the trick..
and...if you had the books already in Calibre before adding the plug-in........remove books from Calibre...shut down, reboot and try again.
Here is the step-by-step setup:
1. Open Calibre and click on the Preferences tab. If it does not show it's because it's on the second line of the menu, click on the arrow at the right end of the menu and it should show it.
2. In Preferences, click on Plugins.
3. On the page that comes up, click on Load plugins from file.
4. Go to where you have the tools plugins and select the K4MobiDeDrm and click on Open. If you like, you can select all the plugins. If memory serves, you unzip the Tools and are left with several zipped tools as shown below. I don't think you have to unzip each plugin separately...I think Calibre can install them from their zip files, but it's been a while since I did this. Calibre will no doubt let you know if this is not the case. But try it using the zip files, as shown, first.
5. When the plugin(s) has been loaded, if necessary, go back to Preferences, select Plugins again and click on the tiny triangle to the right of File type plugins. This will expose the plugins as shown in picture 6.
6. Highlight the K4PC plugin as shown below and then click on Customize plugin. This will bring up a small window where you will input your Kindle serial number [located on your Kindle under Settings].
7. Once you have input your Kindle serial number, you are good to go. Now, when you want Calibre to deDRM your azw ebooks, Calibre should do it without a problem.