Addons are not recognized by the game

Hello. I found no help on this on the web so I might be stupid asking this but how do I add Addons like characters? I have Addons unlocked because I can pick it under Extras, then it says there is no folder. I created a folder called "addons" and added my .pk3-files. Sadly I still got the text No files/folders found. I tried reinstalling didn't help. I'm using Version 2.4 on Windows. Thanks for your help!