Hi there,
 
I downloaded the latest release (4.5 R7) but I used JSON for .Net 3.5 since I am working on a SharePoint 2010 project. After adding JSON references I got a ton of warnings saying some of my dlls were referencing mscorlib version 4.0.0.0 instead of 2.0.0.0 (.Net 3.5 uses mscorlib v2), which caused my solution build to fail.
 
So I downloaded the latest JSON .Net 3.5 release (from 2010), added those references and now everything is working fine. I guess there is some .Net 4 reference on the JSON 3.5 build. Could you please check this.