First, verify that the Unity or Retro drivers are in the OMS folder or FreeMIDI folder (both located in the System Folder of your Mac HD). The driver names will be called "Unity DS-1 OMS Driver", "Retro AS-1 FreeMIDI driver", etc.
If these drivers are missing, you can do a custom install from the Retro or Unity CD. When changing the isntaller from EASY install to CUSTOM install, click "System Files" only. This will place the necessary drivers into OMS and/or FreeMIDI.