I've downloaded a website and in VS express open it through file => open website. When I press F5 to debug I get builderrors:
The type or namespace name 'AspNet' does not exist in the namespace
I guess I'm missing a reference but how come it works on the server? References for these projects can be found in the bin directory right?
Not if the used libraries are installed on the server. You might want to install them on your workstation too, or add the proper references (for example via NuGet) and do a bin-deploy.
On how to find out which libraries you're missing: find class names, not namespace names.