The Shared Media option you see is for media sharing using win7's built-in upnp server - It'll let you stream music/video from your pc to your N900 via wifi, but won't let you arbitrarily transfer documents back and forth.
In order to do that, the most straightforward method is to handle the transfers at the computer side, rather than the n900. The most simple method that comes to mind is to install the openssh server to your N900, then use an SCP client (such as WinSCP) to connect to the device wirelessly over your wifi network, and youll have access to a full explorer-style folder structure of your n900, allowing you to transfer content back and forth as needed.