You have to find the STARTUP directory.
And then follow this: https://github.com/plantuml/word-template
1- Open Word
2- Alt+F11 (to open macro editor)
3- Ctrl+G (to open immediate window)
4- type: ?Application.StartupPath (to know the instalition path of Word, "c:\users\xxxxx\appdata\local\packages\microsoft.office.xxxxxx\localcache\roaming\microsoft\word\startup" in my case) xxxx is hiding my stuff
5- go to that path and follow the second link
Note: .jar file is here http://plantuml.com/en/download