Hi LoveMusic: Don't try to add/enable this visualization plugin via the Plugins/Add Plugin dialog; if you did, uninstall it, so it doesn't show up in the Plugins panel. Then, let's start clean:
1. Go to the MusicBee/Plugins folder in Windows File Manager/Windows Explorer and delete, if still present, vis_classic.ini, vis_classic.dll, and vis_classic subfolder. Then, restart MusicBee.
2. Exit MusicBee. From the original vis_classic10.exe installer, use 7-Zip to extract the same components again--vis_classic.ini, vis_classic.dll, and the vis_classic subfolder--directly into the MusicBee/Plugins folder.
3. Start MusicBee; in the menu View-->Visualiser, the choice "classic" should appear. Select it. Now you can place it using the Arrange Panels/Available Elements dialog, where it will appear as "visualiser:graphic" in the selectable elements box. It will not appear at all in the Preferences/Plugins panel.
Then experiment with its settings as described in the above thread.