The package source named 'NuGet' is either invalid or not available

I have installed Nuget 1.2 yesterday, and today, while I was trying to install Entity Framework package, I have encountered the following problem in the Package Manager Console:

PM> install-package entityframework

Install-Package : The package source named 'NuGet official package source [https://go.microsoft.com/fwlink/?LinkID=206669]' is either

invalid or not available and thus is currently unreachable.

At line:1 char:16

+ install-package <<<< entityframework

+ CategoryInfo : NotSpecified: (:) [Install-Package], Invalid开发者_开发问答OperationException

+ FullyQualifiedErrorId : NuGetCmdletUnhandledException,NuGet.PowerShell.Commands.InstallPackageCommand

I can surft the internet and I don't think anything is preventing Visual Studio to access the internet.

Actually, there was something that was preventing Visual Studio from accessing the internet. Seems like VS had some problems with my proxy server.
So I used a solution that has been introduced in here and everything works fine now.

Same issue happened to my setup. However, i found that VS 2010 uses proxy settings from Internet Explorer. I have removed proxy settings from IE and it was successfully connecting.

To correct this problem permanently, a fix from Microsoft must be used

This is the name of the file KB2581019(VS10SP1-KB2581019-x86.exe)

While this link is active you can download path correction for it.

http://connect.microsoft.com/VisualStudio/Downloads/DownloadDetails.aspx?DownloadID=38654

It's solved!