TextFX menu is missing in Notepad++

It should usually work using the method Dave described in his answer. (I can confirm seeing "TextFX Characters" in the Available tab in Plugin Manager.)

If it does not, you can try downloading the zip file from here and put its contents (it's one file called NppTextFX.dll) inside the plugins folder where Notepad++ is installed. I suggest doing this while Notepad++ itself is not running.

For 32 bit Notepad++ only

Plugins -> Plugin Manager -> Show Plugin Manager -> Available tab -> TextFX Characters -> Install.

It was removed from the default installation as it caused issues with certain configurations, and there's no maintainer.