I have recently upgraded from VS2012 RC to RTM, and since the upgrade to RTM, I can no longer access nuget. I receive the following error in the Manage NuGet Packages window.
Check your system environment variables for http_proxy and https_proxy. NuGet looks at those first if they exist. If those proxy environment variables begin with https, NuGet will fail. Fix them and restart your computer (restarting Visual Studio did not work for me).
You can verify this by opening the Package Manager Console:
Get-Childitem env:http_proxy
Get-Childitem env:https_proxy
If you want to quickly get it working without restarting your computer, manually set the proxies in the Package Manager Console:
$env:http_proxy = "http://your.proxy.com:1234"
$env:https_proxy = "http://your.proxy.com:1234"