I'm using a 6 year old laptop with Windows 10, all updated, with the integrated Intel 4000 graphics chip, also updated driver.
I put the Milkdrop folder into the Plugins folder, and then (and this is where I think I screwed up) I added the dll file using MB's "add visualizer" option. I've since removed the Milkdrop folder and tried to redo it, but no luck. The path is wrong in the second image because I initially set the MD folder into the plugins folder, but I put the MD folder in both plugin and root directory locations just to make sure MB would find it.
I also noticed after dropping the visualizer plugin "Old Sonique" into the MB plugin directory, (I restarted MB), MusicBee isn't seeing it when I go to View>Visualizations. It's not there.
After an evening of troubleshooting, I was able to diagnose this issue, as I was having it on my Windows 10 system. This issue seems to occur only if you are unable to have administrative rights to access said MusicBee plugins folder. For me, I had an issue because I couldn't modify my Windows App Store folder of MusicBee. I was able to bypass this issue by downloading the non-App store version, and could use my MilkDrop plugin just fine that way.