Ethereum launched today just before noon EST. Here are the steps to follow in order to get it set up on your Mac:
1. Open up the Terminal
2. Type: bash <(curl https://install-geth.ethereum.org -L). You should be prompted for your admin account password.
3. Download genesis.json. Move it to a folder on your computer.
4. Type: geth --genesis path/to/genesis.json console where path/to/genesis.json is the location of the genesis.json file you downloaded.
5. Type: personal.newAccount("PASSWORD GOES HERE") with "PASSWORD GOES HERE" replaced with your password.
6. Start mining by typing: miner.start(8)
Further resources: The CLI instructions, Frontier Guide and Javascript commands.