SQL Server 2016 Plugin's not working
Vendors will need to catch up, as the add-in model and API have changed. This will lead to better extensibility, but for now you'll need to wait for the vendors to do this work. There is usually not much advantage to doing all of that work while the product is still in beta and the add-in model is still changing (the version of SSMS released on Friday is on a completely different VS shell and is radically different, which vendors knew would happen, and that would have rendered any work done against earlier RCs and CTPs a waste). 

Please wait for RTM before expecting all vendors to have perfect integration. If you need these add-ins in the meantime, keep a supported version of SSMS installed too. 

Or, in the case of Plan Explorer, use a simple workaround - all our add-in really does is provide a shortcut to right-clicking a plan, saving as .sqlplan, and then double-clicking in file explorer or using file/open in Plan Explorer. Or better yet, generate actual plans inside Plan Explorer, where you get much more useful metrics anyway.