The tcltklib.so is not included in the latest oneclick installer.
Try this :http://www.fxruby.org/doc/examples.html
Example:

#!/usr/bin/env ruby

require 'fox16'

include Fox

application = FXApp.new("Hello", "FoxTest")
main = FXMainWindow.new(application, "Hello", nil, nil, DECOR_ALL)
FXButton.new(main, "&Hello, World!", nil, application, FXApp::ID_QUIT)
application.create()
main.show(PLACEMENT_SCREEN)
application.run()


James Calivar wrote:
> Hello,
>
> I'm trying to run a very simple Ruby script that employs Tk:
>
> ## BEGIN FILE ##
>
> require 'tk'
>
> root = TkRoot.new {title "File to Open"}
>
> TkLabel.new(root) do
>   text 'Please enter the name of a file to open: '
>   pack('padx' => 15, 'pady' => 15, 'side' => 'left')
> end
>
> Tk.mainloop
>
> ## END FILE ##
>
>
> Unfortunately I get the following error when it atempts to execute:
>
> "C:\ruby\sample\james>ruby tk_test.rb
> tk_test.rb:1:in `require': no such file to load -- tk (LoadError)
>         from tk_test.rb:1"
>
> Can anyone help?  I am using Ruby 1.8.4 which I compiled myself using
> cygwin32.  I am running on XP.  I've edited the Setup file in
> C:\ruby\ext\Setup to enable the tcltklib and tk lines, rebuilt, and
> still no go.
>
> Interestingly, I had the same problem with the extension "WIN32OLE"
> earlier yesterday, but after I copied over some lines from some sample
> files, it started to work (which bugs me because my file was verbatim,
> unless there was some hidden chars that I was missing or something
> weird like that).  I had previously rebuilt after altering the
> C:\ruby\ext\Setup file to enable Win32API and win32ole, but that didn't
> work until I copied over the sample file.
> 
> Thanks in advance,
> 
> James