I'm using Notepad 7.6 with "Plugin Admin" and I could not find XML Tools.
I had to install it manually like @some-java-guy did in his answer except that my plugins folder was located here: C:\Users\<my username>\AppData\Local\Notepad++\plugins
In that directory I created a new directory (named XmlTools) and copied XMLTools.dll there. (And I copied all dependencies to the Notepad++ directory in Program files.)