Quick test. Create a folder in add-ons, call it load_test, and put both an empty description.txt and an empty client.cs in it.
Now, does IT show up in the list? If so, you have a problem with your add-on. If not, you have a problem with where you put or what you named your files.
First, can you see all file extensions? If not, then the OS may be hiding a .txt, so you actually have a client.cs.txt, but with the OS hiding it, it appears to be just client.cs. Second, do you even have the right add-ons directory? With the switch to the launcher, the actual folder where everything goes has moved, so you might be putting everything in the old location (if you already had it installed before the launcher), so it doesn't even exist as far as the game is concerned.
Or you may simply be looking for a client add-on loading with the server ones, or a server one loading with the client ones.
