I am trying to install this plugin but when I create the configuration file and start up MusicBee I get the following error.
An error occurred during Museexmatch startup: Access to the path 'C:\Program Files (x86)\MusicBee\Plugins\museexmatch.conf' is denied.
The plugin can't write to the config file, this issue is exclusive to this plugin. The plugin gets a user token from Musixmatch and saves it into the config file.
You can install the plugin into %AppData%\MusicBee\Plugins as @boroda have already suggested, this folder is writable. You can simply unpack the zip there and restart MusicBee (but you can't have dlls in both places so remove them from Program Files first).
As an alternative workaround you can either start MusicBee as admin or change permissions for the Plugins folder (RMB on the folder, Properties tab, Edit..., Users group, Allow full access with the checkboxes). You can do the same for the config itself.
I think I've changed permissions on my side before even thinking of developing this plugin and that's why I had not encountered the issue myself.