Firstly, I would strongly advise against using the IAC driver at all. It is known to have various issues and I have solved multiple reported Mac Pd midi problems on this forum by simply convincing people to completely deactivate IEC in the Mac midi prefs an stop using it altogether (you can search my posts on here for details on the "why").
You do need a third party app to interface that unfinished Mac midi implementation. I would recommend MidiPatchbay which I can personally vouch for as I use it all the time, or possibly MidiPipe, which I have less experience with. Like IEC, you can use those programs to create all the virtual midi ports you need as well as do much, much more such as port/channel rerouteing, translation etc.
Now that said, there's a certain trick you need to know to get it to work with Pd. You can presently only get access to the first 9 of Pd's internal midi ports with Mac OS, but they will not reliably connect to anything beyond the first nine ports on the Mac's midi port list. For this reason you must first make sure that you start out with all midi ports deactivated, then create/activate your nine virtual ports. Then (and only then) you should start Pd and connect it to the nine ports you created. You can then activate all the midi ports on your system and use the third party midi app to route to any port you like. You have to do it in exactly that order or it won't work.
I am pleased to report that there is currently some headway being made in fixing the various long standing midi problems in Pd -- Hopefully in the near future you will be able to connect Mac Pd midi directly like any other midi app without having to go through any of this redundant silliness at all.