Is it possible to save a service on a PC and then transfer it to a Mac? We weren't able to do this.
Yes this is possible. You have to install OpenLP on both, the PC and the Mac. Then just save the service and open it on the mac.
If you are not sure how to do this, you can read the manual: http://manual.openlp.org/creating_service.html#new-open-and-saving-the-service
If you still have problems, you have to tell us where exactly these problems occur.
It's not exactly that easy though, while most service items are transfered from one machine to another, themes are not, so you need to recreate all of the themes from one machine on the other, or you need to export them from the one machine and import them on the other. Even having done this, font's from Windows may not exist on MacOS, so you may need to fix-up the fonts, because the font descriptors may be different.