2 answers
This site has a pretty good explanation of how to install ANT and the Force.com Migration Toolkit on your system. http://www.salesforce.com/us/developer/docs/daas/. In particular, see the "Installing the Force.com Migration Tool" section and prerequisites. Alternatively, you can install homebrew, which is kind of a package manager for OS X, and then install ANT using brew install ant. See the top answer on here for more info: http://stackoverflow.com/questions/3222804/how-can-i-install-apache-ant-on-mac-os-x
