Which ArcGIS 10 product do you have installed? If you don't have ArcGIS Desktop 10 installed then I doubt the ArcMapUI assembly was installed since that assembly is specific to the ArcMap desktop application. The default location is \Program Files\ArcGIS\DeveloperKit10.0\DotNet. If you have everything installed correctly, it should show up in the Add Reference dialog under the .NET tab. I believe you also have to have the target framework for your project set to .NET 3.5.
