To install this plugin just place its directory (highlightex) 
into Tkabber's plugin directory. Most likely it is either 
~/.tkabber/plugins
or
X:\Documents and Settings\YourUser\Application Data\Tkabber\plugins
where X: is the drive your system is installed on.

So, you'll have something like this:
~/.tkabber/plugins/highlightex/highlightex.tcl
or
X:\Documents and Settings\YourUser\Application Data\Tkabber\plugins\highlightex\highlightex.tcl

After that you have to restart Tkabber and go to 
Customize -> Plugins Management and enable the plugin 
in order to get its functionality.
