Customization of keys

I tried to customize the keys in MPC-HC (View-Options-Keys-Fullscreen), I can clear it but doesn't allow to apply the new set for that key, not even putting the old one, it doesn't accept any set of keys to change the default one, not even the default itself.
I'm speaking specifically "fullscreen" key, by default is "F11", I want to change it to "Shift+Enter" but it doesn't allow the change, and if I erase it, I cannot even put "F11" back, it doesn't allow it either. The only way, after deletting the "F11" I have to put it back, is pressing the "Reset Selected" button.
I run the MPC-HC on win7, but in other computers I can customize the keys in any version of MPC-HC, I cannot understand why.
Can you tell me where MPC-HC keeps the configuration settings, so I can erase that file to create new one, just to check if that way I can change it ?? I tried deleting the registration and nothing happened.
Last time I had to format the hard disk to be able to change the keys, but afterwards I don't know what happens that it doesn't allow me anymore changes.
Thanks.

You can reset the settings with the Codec Tweak Tool.

What EXACTLY is happening?
Once you click on a key, it should visually show that it is selected and show a blinking | indicating that you can type a new key binding. Simply type the new key and click outside of the selection to apply.