SSDT doesn't support SQL Server 2008, you have to use BIDS, which pre-dates SSDT. The deployment methods are also quite different, as there's no support for SSISDB. Considering that SQL Server 2008 is completely unsupported, the real solution here is getting the old instance upgraded to a supported one.
Thom~
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.
Larnu.uk