Ray and others... after my problem with the re-install of everything, I found that two of my computers were installed fine, and two, no matter what I did would not add Dreamweaver CC 2015 to the panel. I tried dozens of tries, etc. In a last effort I called Adobe, and had my case escalated to a senior member of the Dreamweaver team. After 2-3 escalations of the call I spoke to a person who stated that Adobe was completely doing away with the extension manager. Current updates to the software will now prohibit it from showing up in the extension manager, and the ability to even download the EM will soon go away. He stated that in the cases where it is currently working, that there must be fragment files in the users system where it is working that is allowing it to happen. He assured me that in the near future as the CC 2015 apps update, all connectivity to the extension manager will cease to work. So basically, if you have it installed now you should be good, but if you need to do a complete reinstall, or a new install of CC 2015, your extensions will no longer work with Extension Manager. He said support for CS6 Extension manager will remain functional.
That being said.. he walked me through the work around using the command-line to install the extensions. He said at this point, that was my best and only option for installing. He said this method would continue to work in the future, and soon be the only way to install third-party extensions (other than those that participate in the add-on program on their site).
The link he gave (and then walked me through installation with) is here : https://helpx.adobe.com/x-productkb/global/installingextensionsandaddons.html
It is worth noting.. he had me completely uninstall the extension manager CC before using the command line tool. Not sure if it would affect the install if it was still there, but worth mentioning so that if anyone else chooses to install this way. He mentioned that I need to uninstall the extension manager on all machines where I'm using the extensions (even the ones which are working fine through EM) and reinstall using the command-line method. He stated that as the apps update soon, use of them though the Extension Manager may cause them to fail later. So just followed the instructions and seems to be fine now.
After working on this for 4-5 hours last night, and half my day today.. pleased to say with a new install of CC2015, and a fresh download of the WA extensions, this method worked perfectly for me. All is up and running when no other solution (other than the MX) seemed to be possible for me.
Ray.. might be a sign that eventually you all have to set up the installers to work outside the EM now.. according to Adobe, the current luck a few have had will soon stop working.
Thought I'd share.. hope it helps. I'm finally up and running and have two more systems to use the command-line method to install on.