Install a new DigiByte wallet and open it up.
Once you have located your existing backup file shut down the new wallet that you just opened.
Make sure you are able to show hidden files on your operating system so you can see the DigiByte data folder.
Open Terminal found in Finder > Applications > Utilities.
In Terminal, paste the following: "defaults write com.apple.finder AppleShowAllFiles YES"
Hold the 'Option/alt' key, then right click on the Finder icon in the dock and click Relaunch.
Navigate to the DigiByte data directory folder and find wallet.dat. By default, on OS X this is in ~/Library/Application Support/DigiByte/wallet.dat
There will already be an existing wallet.dat file from the new wallet you just installed. Delete it and replace it with your old wallet.dat backup.
Restart your wallet and let it finish syncing. If your funds do not appear you may have to restart the wallet with the -rescan or -reindex commands.