issues for NuGet after migrating a c# project

Hi,

i'm trying to migrate a progect into my visual studio 2012.

On the Manage NuGet Packages, I have this issue:

"Some NuGet Packages are missing from this solution. Click to restore from your online package sources"... but when i click on the restore button, it appears this issue:

 "an error occurred while trying to restore packages: Unable to find version '4.1.0-alpha-121112' of package 'Microsoft.AspNet.WebApi'. "

i'm using .NET Framework 4

How can i solve?

Thanks in advance....