You're working in BIDS 2008 but are trying to reference the DLLs for SQL Server 2012. See Using SSIS BIDS with Visual Studio 2012 for ways to work with SSIS for SQL Server 2012, if that's what you're trying to do.
In BIDS 2008, you want the SQL Server 2008R2 DLLs, located by default in C:\Program Files (x86)\Microsoft SQL Server\100\SDK\Assemblies. (Note that the VSTA project should have those references set automatically when you edit the script for the first time.)