I noticed when i was setting up my MIDI devices on Pd 0.43.4-extended using a Macbook that if the IAC driver port name was shorter than the phrase 'IAC Driver' then it would append some letters from that name on the end of the device name, causing it to not get recognised as a valid input.
As example, i had a port called 'PD Out' and it was showing up in the Pd MIDI settings dialog as 'PD Outer' or something, the last two letters being the 'er' of IAC Driver. This issue was solved by making my device names longer.
Thought i ought to post it here in case it's a bug or whatever. It probably only applies to Macintosh OSX and the IAC driver settings. USB devices that provide their own name are fine.