If you try to drag tables from the Server Explorer of Visual Studio 2008 to the designer surface of the Link-to-SQL designer you'll probably be surprised to see the error message stating:
"The selected object(s) use an unsupported data provider"
However you can use SqlMetal.exe to generate the .dbml file manually (obviously: command line instead of drag & drop).
If you get that nice error message change your solution configuration to target 32-bit (x86) using configuration manager.
For more information read this.