Did you add a password to your wallet? If you didn't the instructions below might work. I tested them on windows because I don't have a mac. However, I think multibit uses the same format for wallet files on both windows and mac.
Although any hex editor should work the one I linked to in the quote has a mac version,
Since version 0.4.0 multibit classic wallet files are in a format called Google protocol buffer format (protobuf). Open a file with a .wallet extension in a hex editor and look for the following sequence of bytes 08 01 12 20. The next 32 bytes after that should be your private key in hex format. After you get your private key in hex format you can convert it to a normal format using an offline copy of the bitaddress website.
I tested this on an unencrypted wallet (one with no password) in multibit version 0.5.1.6 and it worked for me.
This is the hex editor I used, although any other is probably sufficient.
http://www.wxhexeditor.org/home.phpThis is the bitaddress website. I advise you not to directly paste private keys into it.
https://www.bitaddress.org/Instead look for this link at the bottom of the page and use it to download a zipped copy you can run offline.
https://github.com/pointbiz/bitaddress.org/archive/v3.3.0.zipThis is my multibit wallet file opened in the hex editor with the bytes 08 01 12 20 that precede a private key highlighted.
This is the 32 bytes of a private key (in hex format) highlighted.
This is the 32 bytes of a private key (in hex format) copied directly from the hex editor into notepad.
This is the 32 bytes of a private key after removing the spaces in notepad.
This is the private key copied from notepad and pasted into an offline copy of the bitaddress wedsite. Click the view details button to get the private key converted to normal formats.
This is my multibit wallet's address 1F84fkbMng6dJpGZmtycRbUe72B7XSYbeT shown on the right hand side of bitaddress. Every raw private key can convert into two different bitcoin addresses, which is why there's two.
This is my multibit wallet's address 1F84fkbMng6dJpGZmtycRbUe72B7XSYbeT shown in multibit.
You can import your private key into the wallet of your choice to get control of yours coins back. Electrum's a good choice.
If you install electrum you can use these instructions to import your private keys from multibit into it.
http://docs.electrum.org/en/latest/faq.html#can-i-import-private-keys-from-other-bitcoin-clients
