2. Next, download a plugin to the same directory that you have the UNetbootin executable in.
3. Then, run the UNetbootin executable, and if all went well, the plugin should be loaded and you should see the customized version, such as this:
If something isn't working correctly, try renaming the plugin to "unetbootin_custom.qm" or "unetbootin_custom_en.qm" (<-- substitute en with your locale here), copy the plugin and UNetbootin executable to your desktop, make sure there aren't other ".qm" files on it, and rerun the UNetbootin executable.
If this procedure works with the standard plugin example but not with the one you're using, it's an issue with the plugin itself, not UNetbootin; if you created the plugin please refer to the guide for Building a UNetbootin Plugin.