Mvc and error after installing NuGet package

hi
i start learning MVC. my pc not connect to internet.
installing EntityFramework and the other nuget package, sometimes is very dificult.
but after many tries for install package, another error will be appeare.
after install EntityFramework to my project,after close project, it don't load again.
this error will shown:

.csproj : error  : The default XML namespace of the project must be the MSBuild XML namespace. If the project is authored in the MSBuild 2003 format, please add xmlns="http://schemas.microsoft.com/developer/msbuild/2003" to the <Project> element. If the project has been authored in the old 1.0 or 1.2 format, please convert it to MSBuild 2003 format. 

thanks