I have installed the eID Middleware, but the plug-in doesn't load in FileMaker Pro.

You have probably installed version 4.x, the most recent version of the eID middleware.
Version 4.x is currently not supported by the the eID and SIS plug-in.

You can download the o lower version from:
http://code.google.com/p/eid-mw/downloads/list

Download the version 3.5.6.