I reply to myself:

I have found two essential info in tcltk.rb, which helped to use
BWidget: there is a TclTkInterpreter class, and it has an 'eval' method.

So, this code works:

require "tcltk"

$tclInterp=TclTkInterpreter.new
$tclInterp.eval <<END
    package require BWidget
    ScrolledWindow .scr
    set t [Tree .scr.tree -padx 0]
    .scr setwidget $t
    pack .scr -expand 1 -fill both
END

.... and so on.

That's cool, but it would be great if I could get more info about the
tcltk module. With this information, now I can write tcl/tk programs in
ruby, that's cool, but it doesn't have much sense. :)  If I could bind
ruby and tcl variables, for example... Or doing other things in ruby
like with 'tk' module...

Bye:
Circum