There was a recent issue where we had Visual Studio 2010 and wanted to add an Elmah package to our project but found that when we went to add it, there was an authentication error.
The solution to fix this was to add the TFS server to the authorised sites list in internet explorer.
- Open Internet Explorer
- Internet Explorer
- Security tab
- Trusted Sites -> click sites button
- Add your TFS server address into the sites location.
- Save and Close IE.
Try to add NuGet package again and it should work.