I noticed the same thing with the grayed out checkbox. My guess is that this is a bug in AR 10. In my case, I wanted to disable the AR plugin and couldn't do it from AR 10. So, I opened AR 9.4.1 (which I still have installed) and was able to do it there (it said that AR 10 was the plugin to be used). If you have that previous version, try going to Adobe Reader > Preferences > Internet and see if you can successfully enable it. Otherwise, you can download the AR 9.4.1 update here
. It's possible that you will have to install AR 9.1 first (if you don't have AR 9) and you can get it at the same place.
EDIT: Evidently, I was writing my reply while you were editing your original post.
Edited by jchuzi (12/05/10 12:35 PM)
macOS 10.15.5, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365