As usually, copy this directory under the ~/.tkabber/plugins 
directory so that you get a hierarchy like this:

  ~/.tkabber/plugins
  ~/.tkabber/plugins/boldnicks/
  ~/.tkabber/plugins/boldnicks/boldnicks.tcl
  ~/.tkabber/plugins/boldnicks/README
  ~/.tkabber/plugins/boldnicks/INSTALL
  ~/.tkabber/plugins/boldnicks/msgs/
  ~/.tkabber/plugins/boldnicks/msgs/ru.msg
  
Restart Tkabber, to get the plugin loaded.