It's called "More Where That Came From - Diamond City Radio Edition"

Found on nexusmods.

Step 5 of the manual install instructions say "Activate the More Where That Came From Diamond City.esp"

Where do I find "More Where That Came From Diamond City.esp"?

That ESP file should be in the ZIP archive you downloaded from the Nexus website. You need to extract that file into your Fallout 4\Data directory, and then go to your C:\Users\\AppData\Local\Fallout4 folder, open up the plugins.txt file, add the "More Where That Came From Diamond City.esp" to the bottom of that file, save, then right-click the plugins.txt file and click Properties, then make sure that "Read Only" is checked.