Flash installed, last version,activated, but no found on this site

The behavior you're experiencing is the result of a combination of things:

• Your browser is preventing Flash Player from operating normally by blocking it by default
• The website is written in such a way that when Flash Player is blocked, there's not a way for you to enable it

The content provider really needs to fix this (and in reality, needs to migrate to native web technologies like WebRTC, which provide conferencing functionality without the need for plugins). 

Short of that, you could experiment with multiple browsers.  Each browser vendor implements their Flash blocking stuff a little differently, so you might find that one works better than the others.

Aside from that, it might be time to look for a better competing service.  There are plenty of options in this space which have already made the jump to current technology.

Ask the site provider what their plans are to replace Flash by next year. Flash Player is going away soon, it is official, and officially it will get more and more annoying to use it until then.