To my knowledge the plug-ins are not yet supported for WebLogic 12c.
"For information on WebLogic Server 10gR3 (10.3.0) and 11gR1 (10.3.x) Web Server Plug-In support, refer to My Oracle Support Doc ID 1111903.1 on https://support.oracle.com"
- New plug-ins will be released for WebLogic Server 12c.
- The location for the WLS 12c plug-ins will be released when they are available.
Instructions on configuration (note they come from the WebLogic 10.3.5 documentation) can be found here: http://docs.oracle.com/cd/E23943_01/web.1111/e14395/nsapi.htm#BGBFCGBJ
- The latest plug-ins can be found here: http://www.oracle.com/technetwork/middleware/ias/downloads/wls-plugins-096117.html
The documentation for WebLogic 12c only has instructions for the HttpClusterServlet: http://docs.oracle.com/cd/E24329_01/web.1211/e24425/setup.htm#CLUST466