G Cast extension does not appear after installing it
I made a vid about what happens to me https://dl.dropboxusercontent.com/u/84308794/ShareX/2016/10/2016-10-25_19-06-39.mp4 Edit: i should also add that that happened on all my PCs and that it persists even if i purge the hole vivaldi folder in %appdata%/local
Hard to say what went wrong for you. For me the extension installs successfully, it appears in extensions and I have a button for it in the addressbar. What version of vivaldi are you on?
Also I just noticed you have a ton of extensions already running. Maybe try and disable all other extensions and then try to install google cast again. If it works you can try and find out what extension is interfering with it – if that's what's really the problem. Just a blind guess.
Pretty sure Vivaldi has a limit at present, concerning how many extensions can display on the address bar at one time.
Same problem for me but i only have 1 extension installed.