Why when I boot up DP does it complain of not finding a valid FreeMIDI configuration when I know I've got one?

Usually this means the FreeMIDI preferences have become corrupted.

1) Quit out of DP/FreeMIDI
2) Within your System Folder -> Preference folder, find the FreeMIDI Preferences and FreeMIDI Setup Preferences.
3) Drag both of these files to the trash and empty the trash.
4) Boot up the FreeMIDI Setup Application and keep clicking 'Continue' or 'Done' until your at a blank FreeMIDI configuration window (or if using a USB interface, a window where only the interface is shown).
5) Go to the File Menu, choose 'Open', and select your FreeMIDI Configuration File (or whatever you might have re-named it) from the window to reload your studio config.
6) Go to the File menu again and choose Save.

DP should find the FreeMIDI config happily now.