A very important step is to change the System Path to include the directory where the DLLs for the proxy plugin are located and then to restart the Windows Server. If the windows server is not restarted, IIS won't pick the directory location after setting the system path
If the System Path is not set correctly then you might get the below error in the Proxy logs
Thus, in order to avoid the above error you need to make sure that the System Classpath is set with the directory where all the plug-in libraries are located. However in order for this system property to take effect on the IIS server the box needs to be rebooted.
Note: be sure this is added to your PATH as well. This worked for me.