As of Notepad++ version 7.5, plugin manager is no longer shipped with Notepad++
From the Notepad++ release notes:
You may notice that Plugin Manager plugin has been removed from the official distribution. The reason is Plugin Manager contains the advertising in its dialog. I hate Ads in applications, and I ensure you that there was no, and there will never be Ads in Notepad++.
A built-in Plugin Manager is in progress, and I will do my best to ship it with Notepad++ ASAP.
If the above doesn't put you off, and you want to proceed and install the plugin manager anyway, it looks like there's a GitHub repository for nppPluginManager - though I haven't personally used it, so cannot comment on it's validity.
The nppPluginManager installation instructions state:
To install the plugin manager, simply download (links below) the .zip, and place the PluginManager.dll file in the Notepad++ plugins directory, and the gpup.exe in the updater directory under your Notepad++ program directory. (e.g. "C:\Program Files\Notepad++\updater")
In fact, if you prefer, you can just add the PluginManager.dll to the plugins directory, then do a reinstall of Plugin Manager from the plugin itself, which will place the file in the right place! Of course, if you're already using an earlier version of the plugin manager, you'll be able to just update from the update tab (or when you get the notification that the update has happened).
The GitHub repository also contains the latest release.