WebM video plays directly in your web browser using HTML5. No plug-ins are needed, but you must install a modern web browser that supports HTML5 and WebM.
For Windows Media Player 12 on Windows 7 or 8, install the WebM Media Foundation Components. Though the components refer to Internet Explorer 9, installing them will add system-wide WebM support in applications that use Media Foundation.
For earlier versions of Windows Media Player, follow the instructions for installing the WebM DirectShow filters. As indicated, be sure to also install the Vorbis audio filters.
Ask and answer questions on the webm-discuss list.