If you can't get to the bottom of this, you can bypass the package installer and do it manually if you are ok with doing low level stuff (and with the risk of messing thngs up
).
Instead of clicking on the plugin, do a right click and save the mjp file on your disk. Open it with an editor and you will see which zip file you need to download, and other normally obvious directions on where the zip file should be unpacked and if the dll need to be registered with regsvr32
Here is a typical package. You can see where the zip file is, that it should go in the plugin directory and that it should be registered
[Package]
Name=WAV (PCM)
Action=UNZIPDIR
Version=2.0.26
URL=http://www.musicex.com/mediajukebox/mj_data/plugins/in_wav.zip
[Action]
File1=in_wav.dll
File1Action1=COPY_PLUGINDIR
File1Action2=REG_WA_IN
Claudio