Backing up a Mac wallet is very similar to backing up a Windows or Linux wallet. In all cases, your goal is to find the "wallet.dat" file created by your wallet app and to copy it somewhere safe. The only difference between Mac, Windows, and Linux is where your wallet.dat file is stored.
Before getting into the mechanics of backing up your wallet, please let me remind you that it is very important to encrypt your wallet, if possible, before backing it up. If you make a backup of an unencrypted wallet, and somebody else gets hold of it, it's no problem for that person to send your coins anywhere they want.
With that in mind, the easiest way to back up a wallet on any system is to use the backup functionality in the wallet app. Most wallets have a "Backup Wallet..." option on the File menu; when you click that, the wallet will save a clean copy of your wallet.dat in the location of your choice. You also have the option to name the backup file something other than "wallet.dat"; that's handy if you want to embed a timestamp or add the name of the coin to the filename.
The only problem with using the Backup Wallet functionality is that, if you have a lot of wallets, it can be a pain to have to open every wallet and back it up. Luckily, you can also back up a wallet by finding its wallet.dat file and copying the file to some other location.
If you'd like to use a GUI to do this, it's possible to do it with Finder. However, you'll have to do a small hack to show hidden files first. To expose hidden files in Finder, open a Terminal window and type:
Code:
defaults write com.apple.Finder AppleShowAllFiles YES
Note that if you want to hide hidden files again later, you can do so by running this command in Terminal:
Code:
defaults write com.apple.Finder AppleShowAllFiles NO
Once you have hidden files exposed, you'll need to navigate to the Application Support folder, which lives under the (previously hidden) Library folder in your home directory. You can do this by opening a Finder window (by clicking on the Finder icon in the dock or double-clicking a drive icon on your desktop), and selecting your home directory (which should be in the left pane of the Finder window--it's the same as your Mac username). In your home directory, double-click Library, and in Library, double-click Application Support.
Now you should see a bunch of folders that contain various preferences and files used by various apps, including wallets. The wallet directories for different wallets are typically named after the corresponding coins, e.g. Bitcoin, Litecoin, etc.
If you open any of those folders, you should see a number of files, including wallet.dat. Before you copy a wallet.dat, make sure the wallet app is closed; then you can copy it the normal way (by using copy and paste, or by dragging and dropping). Just copy the wallet.dat files you want to back up to another location (Dropbox, USB drive, elsewhere on your disk, etc.) and you have backed up your wallet.
Restoring a wallet is just as simple, whether you've backed it up using the wallet app or by hand. Just locate the Application Support folder for the wallet you want to restore, and copy its wallet.dat file into that folder, and you're done.
If you're comfortable with Terminal, you can also copy wallet.dat files into and out of wallet directories using the usual Terminal commands. Your Application Support directory can be referenced at the command line as "~/Library/Application Support" and individual wallet directories can be referenced by name, e.g. "~/Library/Application Support/Bitcoin". Note that when you're using the command line, all directory names are case sensitive.
Questions? Feel free to ask.