getting Tk working with Ruby 1.8.6

Hey all, i've installed ruby using MacPorts, months before : %> ruby --version ruby 1.8.6 (2007-03-13 patchlevel 0) [powerpc-darwin8.9.0] and i'd like to try Ruby/Tk (for Arcadia afterwards) however i get : `gem_original_require': no such file to load -- tk (LoadError) with a simple Tk example. what do I need to do in order to get Ruby and Tk working together on 10.4.11? best, yvon