Open Vivaldi menu, choose File → Tools → Extensions (see pic. 1). Scroll down the page and click on Get more extensions (see pic. 2). You will be redirected to Chrome Web Store. Enter Adblock, or Adblock Plus, in the search form which is on the left of the screen (see pic. 3). Find extension that you want to install and click Add to Chrome directly from the search filter. It will be added to Vivaldi browser.