Referring to previous article on how to encode or decode base64 encoding with URL of http://www.mickgenie.com/encode-or-decode-base64-encoding-with-notepad/, I have found out the default NotePad++ do not come with this plugin and you are not able to done it directly.
To make it done, follow the step as below.
1. Open NotePad++.
2. Open Plugins tab and click on Plugin Manager then expand the Show Plugin Manager.
3. Look for MIME Tools and install it.