ePIX, I just uploaded an archive with dspadpcm, the user guide and 2 simple batch I create to simplify the task: one to convert your WAV files to DSP and the second to convert DSP files to WAV. You can download it here: http://www.mediafire.com/?su86u1w6rmtu6dp.
As mentioned above by bxaimc your audio files have to be 16 bit Monaural WAV or AIF files otherwise it won't work. So first convert them (you can use soundforge), put all you wav files in the same directory as dspadpcm.exe and execute WAVtoDSP.bat.
I never used dsp files but hope it can help you.
Edit: I didn't include any options in the batch so if you want to add more options just edit them with notepad.