I think you have to click on the file(s) to select them first, then rename
I just highlight the file name in the Organizer Properties box and type in the new name and then just click somewhere else on the screen. Works everytime. Just do not try to change the file format this way., e.g., if it is a jpeg format in the properties box, do not try to change it to a psd or some other format. In other words, you just highlight the file name to the left of the decimal point and only change that.
If you are working in Organizer, You have to select the file(s) to get the File -> rename enabled. If you want to rename multiple files in one go, you can use the Process Multiple files option in Editor. In Editor, You can find this option in File -> Process Multiple Files.