Afaik that extension doesn't have an icon to be shown in the extensions bar.
Or do I need it, just visit the web store and carry on ?
Yep, that's it. Just note that Chrome extensions are not officially supported by Opera, so even with that extension there may be Chrome extensions that will not work in Opera.
I tested it on Windows 10 with different combinations of OS DPI, browser zoom, CSS zoom and sidebar width...
I hope this applies to Mac OS and Linux too
If anyone finds any problems, pls report it with your specs (operating system, DPI)