To embed your RebelMouse page on Posterous, follow the example below:Replace 500 with your desired height, and “rebelmouse” with your site name:

<;iframe height="500px" width="100%" scrolling="auto" src="https://www.rebelmouse.com/rebelmouse/?embedded=1" class="rebelmouse-embed-iframe" id="rebelmouse-embed-iframe"><;/iframe>

Where to place it will vary based on the theme, so you'll have to have some knowledge of HTML. Manage Spaces -> Setting -> Customize -> Advanced -> Enable Advanced Theming, and then placing in the right place in the Custom HTML.