FTP is an insecure protocol that should never be used, much less as part of any new development. It is strongly recommended that you leave FTP disabled and access your site with SSH or SFTP.
FTP access can be controlled on a per-member and a per-site basis. Although some people expect these settings to be related, or expect one to supersede the other, they are completely independent. Both the member accessing a site and the site being accessed must have FTP enabled in order for access to succeed.
This is to allow maximum access control granularity in situations where multiple members share access to multiple sites via our adjunct access feature. For example, if you don't want to allow FTP access to your site, and you want to make sure a member with adjunct access doesn't enable FTP access for himself and overrule you.
To enable FTP access for your membership, visit the Profile page and toggle that setting in the "Details" box.
To enable FTP access for your site, visit the Sites panel and select the site by its "short name" to view its Site Information page, where you'll see the FTP setting in the "FTP/SFTP/SSH Information" box.
Note that as of May 18, 2010, FTP access is deprecated. While we have no current plans to remove it, it is disabled by default on new sites and memberships. (And must therefore be enabled in both places if you want it.)