Instructions:
If you have previous version you must first remove old language file, to do so in Notepad++ open "Language/Define your language...", in "User language" box at the top select CK2, select "Remove".
The zip file contains two files - CK2.xml and CK2lang.xml. Place CK2.xml in directory where you installed "Notepad++/Notepad++/plugins/API" (default path would be "C:/Program Files/Notepad++/plugins/API"). Run Notepad++ open "Language/Define your language...", click Import and chose CK2lang.xml. Open "Settings/Preferences/Auto-Completion" and check "Enable auto-completion on each input" and "Function completion".