Although I don't use a Mac I'll try my best to help.
You install downloaded maps like so:
1.) Follow the "read me" file if there is one that came with the download (it would be a little note pad file). Then place the files in the places where it says or just place all the files into the "Exports" folder. (The exports folder is in the main Feral Heart Folder).
OR
If placing the map files into the Feral Heart "Exports" folder didn't make it work right than try placing the files as so:
FHM files go into FH>Media>Terrains
FHO files go into FH>Media>Objects
FHS/FHW go into FH>Media>Sky
Mp3 go into FH>Media>Sounds/Music
Then restart the game.
Once you get back on head over to the Cape of Distant Worlds and you should see the new PokeIsland portal
Hope that helps.