hello and welcome to the notepad++ community.
if you want to add it automatically:
download and install version 7.3.3:
and then download and upgrade it with the latest version:
if you want to add the plugin manager manually to your fresh 7.5.1 install:
you can download it here:
(uni for 32bit and x64 for x64bit installations)
and copy the extracted files to your notepad++ plugins and updater folder.
the plugin manager project isn’t automatically included anymore, because many npp users didn’t like the sponsored commercials in the latest versions of plugins manager.
it is still easy to add it by copying PluginManager.dll into the plugins folder, it remains a downloadable extra option but it isn’t a forced default installation for all notepad++ users anymore.