It's added by default to configure/proftpd/configure.proftpd So it's compiled-in, if it doesn't work for you - then you need to take a look at /etc/proftpd.conf file.
Here is what's in my /var/log/proftpd/proftpd.tls.log file.
I have searched google, but didn't find any pertinent info.
Jun 30 01:03:17 mod_tls/2.2.1[29941]: using default OpenSSL verification locations (see $SSL_CERT_DIR environment variable)
Jun 30 01:03:17 mod_tls/2.2.1[29941]: TLS/TLS-C requested, starting TLS handshake
Jun 30 01:03:18 mod_tls/2.2.1[29941]: unable to accept TLS connection:
(1) error:1408A10B:SSL routines:SSL3_GET_CLIENT_HELLO:wrong version number
Jun 30 01:03:18 mod_tls/2.2.1[29941]: TLS/TLS-C negotiation failed on control channel