If you're using a version of Cubase 13 other than standard/pro version you'll need to place the script files in the relevant location manually.

The script files (novation.zip) are available as a download at the bottom of this page.

Once downloaded, unzip the enclosed folder and move it to the Documents/Cubase LE AI Elements/MIDI Remote/Driver Scripts/Local/

Mac: 

Windows:

Once this is done you'll need to reopen your version of Cubase, the Launchkey MK4 integration should then begin to work as per this guide.