We are aware of issues with installing Add Ons for Flash Professional CC 2015. The Extension Manager team is working to fix the issue. Meanwhile, for all users that need their Add Ons to work with Flash Pro, please continue to use the previous version until we have a fix in place.
And do note that the install instructions specify you can use the advanced installer to retain an older copy of CC while installing 2015 so you can have both. If you do not use that option, 2015 will wipe out older versions of Flash installed by default. So you can check out the new features but still keep your add-ons working.
Damn I wish I'd seen this BEFORE I updated. No extensions is a HUGE deal breaker for me.
Can easily install the older versions of Flash Pro even after you've updated. Follow the instructions here: How to Find and Install Previous Version of Adobe Apps in CC 2015 | Adobe Customer Care Team