Visual Studio : NuGet Package Source requires credentials

Visual Studio 2012 > Tools > Options > Package Manager > Package Sources

I add a new Package Source pointing to a myget.org url :

http://www.myget.org/F/myfeed/

VS > Tools > Library Package Manager > Manage NuGet Packages for Solution...

I am now prompted for myget credentials. Obviously, it will be inconvenient for developers to have to manually enter credentials every time they work with myget packages.

This blog explains how to store credentials in the machine-level NuGet.config :

https://gist.github.com/xavierdecoster/3205826

but after I follow the steps, VS still prompts me for credentials.