I had the same problem as Roland. I could not find WebBrowser in the toolbox, and none of the tips above worked for me.
What I did instead: I added a Button control where I wanted my WebBrowser control. Then I manually changed the code in the Designer.cs file to replace the button with a WebBrowser instance. Once the control is on the form, I can reposition it normally in the GUI editor.
A bit clunky, but maybe this workaround can help others as well.