You can't find it, because Mail is smart enough to figure out what you want to do, and do it, without being told.
TLS refers to the standardized version of SSL. However, in common usage, almost nobody uses the term 'TLS' as equivalent to 'SSL'. Instead, TLS is frequently used as shorthand for connections that start out unencrypted, and then use the StartTLS extension to various protocols to boot the connection to be encrypted.
So, in Mail, when you set the IMAP server port to '143' and check the 'Use SSL' box, Mail says "Oh, he wants me to connect unencrypted, then do StartTLS." The same thing for when you set up the SMTP server, set the port to '25' or '587', and click the 'Use SSL' box.
IMAP port 993 starts encrypted (SSL), then does IMAP inside that. The same for SMTPs port 465.
