If it didn't show up in your addons list, then you did not install it correctly.

I will try removing it, re-downloading, and re-installing.

I'm afraid it still doesn't work. Here's how I installed it:
1. Download the file which is in .zip format
2. Extract the file, making it a .vpk format
3. Put that .vpk file in the addons folder

I've installed plenty of other mods and they work fine. Just to make sure I just downloaded a fireaxe skin and that showed up un the addons list and changed the fireaxe in-game. Sometimes a mod won't work but it would still appear in the addons list, regardless. Could it be that I should not have put it in the addons folder?

Hmm. Well there's three things I know that could be causing the problem:
1) Macs are known to be unable to read vpks with long file names like this one. If you have a Mac, try shortening the file name.
2) Try verifying integrity of the game cache. It'll replace any damaged or missing files in your left4dead2 folder (if you have and desired files, remove them first, then verify because they will be gone after the process).
3) The very unlikely one: a fake copy of l4d2. Pirated versions are known to not work for some mods.

I really hope this helped, because I can't think of any other possible issues. Good luck!

I do play L4D2 on a Mac, but shortening the name of the .vpk didn't work. I tried downloading another concert mod of yours, the Hatsune Miku one, but that didn't work either. My copy of L4D2 is legitimate. As for veryfying my game cache, I've no idea how to do that just yet so I'll postpone this method until I learn how...

On a side note, after some frustration I just decided to download a concert mod from the Steam Workshop created by Devoc called 'Hatsune Miku Concert and Credits', and amazingly, IT WORKED. I have no idea how this could happen, but for now, I'm satisfied with it. But I still really wanna try getting Megurine Luka to work.