Thank you for trying out EmEditor Professional RC.
The Association tab of configuration properties associate file types to EmEditor configurations, but it won’t associate file extensions from Explorer. If you want to associate the file extensions from Explorer, you will need to associate in the Associate with EmEditor dialog box, which you already tried.
I don’t know why your change didn’t affect Explorer’s behavior. I don’t know why Windows own File Association manager will not allow you to change associations either. I will do more research. Meanwhile, do you use an Administrative account on Windows 7? Do you have any system policy that might prohibit changing file associations? Any hints might help.