have you enabled the serial port in h8000fw settings. Read page 141 in User manual, as that will explain the settings that need to be done and where to set them. Also the settings should be set the same on both the h8000 and on your Mac. And if you have set that up correctly, are you sure the serial cable you are using is an RS232 cable, as it must be bi-directional. Also, I am sure you did this already but just in case..check that the serial port is enabled in the Mac's bios and OS.
Try that and let us know how you get on. And try to persevere. It is quite a complex machine and can be quite tempermental at times, but once it is set up correctly it sounds sooo sweeeet 🙂 And finally, you can always ask for help here. The eventide guys are very helpful, as are some of the users here, and most of them know a lot more than I do 🙂