> I'm actually using a Mac with Snow Leopard...
> At the Bash command line: the command tclsh invokes TCL, so it is installed
> on my machine...
> However, in irb: require 'tk' gives an error message...
>
> I'd appreciate any advice...quite frankly, I don't know how to install TCL
> with Ruby gems...

http://mxcl.github.com/homebrew/

$ brew install ruby
$ /usr/local/bin/ruby -v
ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-darwin10.6.0]

$ /usr/local/bin/ruby -S irb
irb(main):001:0> require 'tk'
=> true
irb(main):002:0> root = TkRoot.new() { title "Hello, world!" }
=> #<Tk::Root:0x00000100840fc0 @path=".">
irb(main):003:0> Tk.mainloop()
=> true