tnopen is not an application, it is merely a stub in which to open deja32.exe and automatically connect to a specified host put on the command line, either from a script, or by calling from another application (i.e. setting your browser's default telnet helper to deja32).
To use tnopen, you must first go to the plus program directory and create a link from dejawint.exe to deja32.exe, or alternately, make a copy of deja32.exe in the same dir, and rename the copy to dejawint.exe. This is because tnopen tries to execute dejawint.exe,
which was the old name for deja32.exe
Now on the command line, you can type:
tnopen hostname
Typing tnopen on the command line without a hostname will give program error in XP.
If you want use tnopen as the telnet "helper" app in a browser, it should work after creating the above link. Do not specify a hostname after putting in the tnopen path; in this case the browser will supply it.