a) The EXS24 is built-in to the Logic Application. If it is not there or working, you may have an issue with the installation.
It IS possible to install Logic Studio (1) using Pacifist. The standard mpkg is PPC only but the rest of it works fine.
2) You do NOT need a DigiD interface to use the EXS24. Choose the correct interface (Focusrite) via Preferences:Audio (assuming you have installed any drivers that are needed).