Download the latest Ethereum Wallet from https://github.com/ethereum/mist/releases and install it.
You will find a geth executable in the subdirectory resources/node/geth/ under your Ethereum Wallet installation directory.
Delete your old copy of the blockchain with the command rm -rf ~/Library/Ethereum/chaindata/*
From your Ethereum Wallet installation directory, run the command ./resources/node/geth --fast console .
If this does not work, you may have a firewall issue. geth expects port 30303 for communicating with peers over the Internet. Are you running this from a corporate network?