Ditto to Boroda's experience. Maybe trying to summarize the install will help those who have problems with this. As always, start with the latest version of MusicBee:
1. Close MusicBee. Download the plugin at https://getmusicbee.com/addons/visualizer/368/classic-spectrum-analyzer/
, and extract both the DLL and the folder inside as-is into the MusicBee Plugins folder.
2. Start MusicBee. Go into Configure Layout/Arrange Panels, and look for the choice "visualiser:graphic" in the Available Elements list.
3. Pull the "visualiser:graphic" element over to the left side to either "bottom panel" or "top panel" or "right sidebar," and hit Save. then restart MusicBee.
4. Resize the visualiser window as desired, then drop its menu and choose "classic" from the choices. Restart MB just to be on the safe side.
5. Now if you right-click within the visualiser window you will see the settings menu for the analyser. These can be adjusted in real-time while music is playing. Choose a preset from the Options tab if desired.