You should launch the browser, open the menu (Vivaldi icon) → Tools → Extensions (see pic. 1), or use Ctrl + Shift + E hotkeys. Then scroll down the page and click on Get more extensions (see pic. 2) to download add-ons from Chrome Web Store. Choose the extension that you like and click on Add to Chrome (see pic. 3). This add-on will be added to your browser. However, some extensions may not fully work at the moment because Vivaldi is still in progress.