Hi!
The first part - to move the data from the PC to Mac - is fairly simple. Open OpenLP on both machines and go into the "Tools" menu and click "Open data folder". This will open a fileexplorer in the OpenLP data folder. Now close OpenLP again. Copy the all folders (or just the "songs" folder if you don't care for the other data) from the PC to the Mac (perhaps using a USB stick) and insert it into the data folder on the Mac. The songs are now copied to the Mac.
Now, the challenge here is that while the linked audio files is copied over as well in the song-data folder, the song database constains the old paths to the files, so they will not work.
You therefore have 2 options:
1. Relink all the music files to the songs manually
2. Change all the links in the song database in on go using a tools like sqlitebrowser and some SQL.