I cannot access my client's ftp site from my macbook. What software can I download to enable this?

Try Cyberduck (http://cyberduck.ch/) or Filezilla (http://filezilla-project.org/download.php?type=client).

Or Fetch (http://www.fetchsoftworks.com), one of the oldest and certainly best FTP clients for the Mac. Works beautifully.

If you only need to download files (not upload), then you can use the Finder to connect to FTP servers as well.

If you're having trouble connecting to FTP sites, check out what proxies you have enabled as well as if you're using active or passive mode FTP.

you can use ftp command from terminal, it is pretty cool.