Here's how to back up VF data on a Mac:

Navigate in Finder to /[your username]/Library/Preferences/LDW/Virtual Families. The folder will contain ldwLog.txt and a variety of VirtualFamilies[number].ldw files, for instance VirtualFamilies1.ldw. The number of these will vary with the number of families you have. Now, copy the folder "Virtual Families" to somewhere safe, preferably a flash drive or external drive. (To back up the game itself, just copy VirtualFamilies.app to wherever you choose to back up your files to.)
To copy this data to a new Mac, first copy VirtualFamilies.app to whatever folder you choose (possibly Applications), then run it once. Then, navigate to /[your username]/Library/Preferences/LDW/Virtual Families. Copy the folder "Virtual Families" to the folder LDW. Say yes to the "do you want to overwrite?" message, and you're set! Run VF again to start playing.

Here's how to insert mods on a Mac:

Locate your copy of VirtualFamilies.app in Finder. Control-click (or right click if you have a mouse) on it, select "View Package Contents." Open the folder Contents, then the subfolder Resources, then make a duplicate of the folder "images." Rename it "backup." This is your backup in case anything goes wrong. Then, go to the VF mod forum, locate the mods you want, download them, and copy them to the folder "images." Say yes to the "do you want to overwrite?" message. Restart VF and your new mods will instantly appear! If you don't like the mods, go to the "backup" folder and locate the original image file. Drag and drop it into the "images" folder, restart VF, and your game will be just like it was before you added the mods!

I hope this helps anyone with a Mac. Happy backing up/modding!

We also have official procedures on how to backup and recover savegame files for Mac and Windows here and mod installation for Mac and Windows here.

Oops, didn't know it was already outlined on the forum