Well, that happened to me, and I think the problem is that the plugin requires the 1.2.0 or higher version of MV to work, and if you updated MV but you're using the plugin in a project made with a previous version, the plugin is not going to work.
You need to create a new project and transfer...
Another great plugin, and with that, now Yanfly has more MV plugins in his library that VX Ace scripts! Great job! :D
P.S. : Are you still planning to port the Skill Cost Gold script in Ace as a plugin? Because the poor plugin was always left last in the polls... ;_;