I'll start by saying I haven't used the DSP system - my DIY cnc uses PC control through Mach3 software and parallel port. But looking at the DSP system, and Deskproto CAM software I think your flow would look like this:

1. Draw object in CAD programme (didn't see and CAD ability in Deskproto) - create STL file
2. Use Deskproto CAM software to create the toolpaths (which tool to use, feed and speed, path to cut) and the related NC code (g-code) to create this path - This is the post processing bit
3. Import these NC text files into the DSP (via USB stick??? Not sure)
4. Run the programmes through the DSP to command the machine to move around and cut (I think)

I'm sure someone will come along with a more accurate flow . . . but the above suggests you will need a CAD program unless you are just cutting out existing STL files drawn by someone else.