There's been some forking with the ECP2plugin/assembly, but in the source of the original version, I've actually removed the need to find that file. The code to do that is below, but STSC (responsible for the PVRScheduler version) should know how to fix that.
Code:
System.Runtime.Remoting.RemotingConfiguration.Configure(null, false);
In the time being, if you create a MediaPortal.exe.config file in your MediaPortal folder it should work. I'm surprised that that file is missing, but nonetheless nothing in that file is actually used for this plugin AFAIK.
Sam