Hi, and welcome to the forum!! I always make sure that I AM online, edits don't work offline like they do online. I'm sure you know this but it's bit me before.
Also, I always just click once on the rung I want to edit and then hit "enter" to start the edits, and if they are not real complicated or a safety issue where I actually ought to test them, then I just use "CTRL+SHIFT+F" and hit "enter" when the dialogue box comes up asking me if I'm sure, etc...to finalize the edits.
If you're on line and can't get into edit mode, then maybe you have a trial version or something is not enabled. What version of 5000 are you using? Hope this helps.