Hello Saragis, on the next version of the plugin I am working on a way to check if the plugin is running through the browser to make it easier to troubleshoot. At the moment you could test it locally to see if the plugin is operating properly with http://sockettest.sourceforge.net/
Get the binary version of the Socket Test app extract it in a folder and run the exe. Also start MusicBee. On socket test go to the client tab and on the connect to -> IP address set 127.0.0.1 (for the local host) and 3000 (or generally the port that the plugin uses for connections). Then press connect and go to the message field. In the message field write "<player/>" (without quotes). you should see something like this --> S: <player/>
If it won't connect then for some reason the plugin is not working. One cause could be that the port is in use if you try a different port without success then I have no idea but I could try to send you a version of the plugin with logging enabled to figure out what goes wrong.
If it connects on the other hand it could be an issue with the router. Some routers feature a mode where the isolate the wireless clients so that they cannot access directly other clients on the network, or It could be a firewall issue.
Another test that could be done with SocketTest is to close MusiBee go to the server tab let IP 0.0.0.0 and set port 3000 and then press start listening. After this go to your device and press connect... if everything works ok you should see a connection from your mobile device's IP and a request for <player> on the logging textarea.