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

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

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