Add a custom ASP.NET control to a website
To use custom ASP.NET controls in Microsoft Expression Web, you need to make the custom control available to Expression Web by adding the custom control assembly file (.dll) to a folder named "bin" in the root folder of your site. This enables you to use the custom control in any web page in your site.
You must upload the bin folder and the custom control to your web server in order for the custom controls to work unless the custom controls are installed on the web server.
Alternatively, you can install the custom control into the Global Assembly Cache (GAC) on both your computer and your web server. For information about this advanced method, see Global Assembly Cache in the MSDN library.