You need to enclose whole command into quotes
WinScp3.com thughes:<password>@<server> /command "put c:\localdir\file.txt"

But I find it strange that I do not see any error message in the output. So there may be another problem as well.

Try
put c:\localdir\file.txt file.txt
or
put c:\localdir\file.txt /whatever/path/to/file.txt

i've been through the documentation, but I'm clearly missing something. I want to do a put to my home dir on a server from the windows command line. I can connect, but it does not execute the put...any ideas? here is my session:
++++++++++++++++++++++++++++++
F:\>WinScp3.com thughes:<password>@<server> /command put c:\localdir\file.txt
Initialisation...
Searching for host...
Connecting to host...
Authenticating...
Using username "thughes".
Authenticated.
Starting the session...
Reading remote directory...
Session started.
Active session: [1] thughes@<server>
winscp>
++++++++++++++++++++++++++++

Thanks for any help!