I forgot to mention that you'll also need Visual Studio SP1 Tools for SQL Server Compact 4.0, which allows you to add the data connection in Server Explorer of Visual Studio. It has the tooling to visually design tables as well. I haven't played with the WebMatrix table designer too much, but the designer in Tools for SSC 4.0 looks more like the designer in SSMS, which is what I'm used to.

The Tools for SSC 4.0  can be installed from the Web Platform Installer.

The SSC Toolbox can be installed from the Extension Manager in VS.