You have to use latest version of series 1.x of plug-in which is 1.4.2.5.
From what I see you trying to connect to server using ssh. Native way for
cvs for this is to use :ext: protocol. :ext: protocol assumes that you
managed to establish password less ssh connetion to server and configure
plug-in to use external ssh compatible executable for connection.
Configuration can be made on
http://www.pushok.com/help/cvsscc/index.php?redirect=ui_config4.htm page.
Alternative to :ext: protocol is to use :ssh: protocol. :ssh: protocol is
an extensions of CVS NT client which is shipped with plug-in by default.
When you use :ssh: protocol cvs client tries to use build in on putty
compatible libraries to establish ssh tunnel and use log-in name hard coded
into CVSROOT and cached password (which you will be asked when pressing
Login button) to establish tunnel using plain password authentication. In
case plain password authetication disabled on your server this might be the
problem.
And of course for beggining you can start from something less complicated
and use :pserver: protocol instead.