TNCs have two modes, "command mode" and "data mode". When in command mode the TNC is expecting commands that will be acted on locally and not transmitted. When in "data mode" anything sent to the TNC gets transmitted to your current connection.
TNCs generally recognize a special character that causes an escape from data mode to command mode. To send this escape character use F2 or choose Edit/TNC Escape.
To check the current definition for this escape character choose Options/Packet Options and Port Select and pick the Options tab. The default value is "3", the ASCII value for Ctrl-C. This is the value used by most TNCs. ASCII values for other control characters are 1 for Ctrl-A through 26 for Ctrl-Z and 27 for Esc (Escape). For a complete list of ASCII control character values see TNC Messages