Make sure osu! is closed, navigate to your game directory and open the config file listed with your username and check if the parameter below is set to 1.
"Wiimote = 1"
Save it and then right click the config file, properties and then uncheck Read-Only if it's checked.
I don't know if the Wiimote is supported anymore but the parameter is still present in the configuration file so I'd assume it works.