Windows 8 and Windows Server 2012 does not include .NET Framework 3.5 by default. When you run an application that requires .NET Framework 3.5(for example MSSolve) Windows will automatically trigger to install the feature from Windows Update.
The application must do one of the following for the trigger to occur:
If the computer does not have access to the internet or is using WSUS you can point to a DVD or network location (Enterprises could have the files on a network share and use group policy/registry key to direct clients to it). For additional information see the following:
Note: There is no .NET 3.5 redistributable download available for vendors to include in their applications. Changes in architecture require the installation using one of the methods above