What's the name of the add-on, and can you post what's inside the namecheck file?
Namecheck isn't needed, and is entirely pointless for a practice add-on that won't be released
A few things to do:
Make sure your description.txt and client.cs are correctly spelled.
Make sure your client.cs is actually client.cs, and not client.cs.txt
Make sure the two files are in a subfolder in the add-ons folder
Make sure the subfolder follows the broad naming requirement (Category_Name)
Did you put it in the Blockland folder in My Documents?
Check the console for errors related to your add-on - Either look through the console in-game, or close the game open the console.log file and ctrl+f for your add-on name