Text files can be sent to your current connection by choosing Send/Send File. This feature allows you to prepare a file using a text editor and then send it later.
The contents of the clipboard can be sent to your current connection by choosing Send/Send Clipboard. Many Windows applications allow you to move text to the clipboard with Cut/Copy commands.
Choose Send/Stop Sending File/Clipboard to stop any sending in progress.
If your TNC doesn't seem to be getting every character from a file being sent or you seem to be sending lines faster than your connection can handle them, try setting the "send pause" value. To set this value choose Options/Packet Options and Port Select and pick the Options tab. The Send Pause value defines the time (in seconds) to pause between each line that's sent.