Hi - maybe start with finding the parallel port addresses on the Mac, following directions as seen in this post. I’m not 100% sure how parallel port to USB adapters work, but if the USB is somehow emulating the parallel port, then it may just work with the same code as before. It didn’t for that forum poster, but you should try it! Let me know how it goes.
Edit1: Actually, using the Parallel function library will throw an error because it’s designed for use with Windows and Linux. Will get back to you.
Edit2: Based on this post, these people were running a study that did involve an MRI parallel port connection on a Mac OS X, so I change my mind - do try using the parallel class, as it may just work.