Firstly, worldcraft is completely outdated. Nobody uses or supports it anymore.
You want to get Valve Hammer Editor 3.4 (or 3.5 beta) as well as the Zoner's HL tools.
Both are available for quick download here.
As for your question, press shift+A to bring up the face editor. With it open you will select faces in 3D view, rather than whole brushes.
Welcome to the community!