Once that is loaded, you can now search for WebKit. Knowing that WebKit is a library I prepend my search of WebKit with Lib, so instead of just searching for webkit (which actually turns up quite a few hits), I'll search for libwebkit:
You want to mark the first hit of libwebkit-1.0-1 for installation, then apply. For those who like the command line approach, you can launch the terminal and type:
sudo apt-get install libwebkit-1.0-1
You are now ready to enjoy the new HTMLViewer using WebKit!