Seems to be a FileMaker error. What OS? Installation issue? Access rights? Where is the plugin? (application folder?)
windows 8, FM 14 adv, full admin access and the default application folder is used on the install.
FYI, if you're using the script steps to install, it does not put the file in the application folder, but in a user folder instead:
Help: Installing plug-ins
Also, even though it has nothing to do with the error, but just to be safe, does the plug-in match your client (32-bit or 64-bit)?
that's was it! doh! the plugin wasn't 64bit.