Windows 2008R2 doesn;t install .NET Framework 3.5 automatically, yet when you download it from the MS website, it won't install (you get the error
You must use the Role Management Tool to install or configure Microsoft .NET Framework 3.5 SP1).
What you need to do is install it using PowerShell in a command prompt (running as an Administrator):
powershell.exe -noexit -ImportSystemModules Add-WindowsFeature net-framework
Easy!