Just installed (a few days ago) the latest 4.3 build dated June 6 2014 for 64bit version. I can no longer edit registry values. See attachment for a picture of the error dialog that pops up when I attempt to change a value.
Now, between the time that I installed the latest MC and today, a Windows 7 update was also pushed to my machine and installed. So I'm not 100% sure that MC is the cause of the problem. But if it's not, I don't know where to look to fix it.