First find where your system keeps it's mt.cgi script. You'll need to create a directory here named 'plugins'. If you're connecting to your site via FTP then 'cd' to the mt.cgi directory and 'mkdir plugins'.

Now that you know the directory exists you can put plugin scripts into it. You'll need to create a text file. You'll create this locally on your computer and then use FTP to put it up on your MT site. I created a file named pingentry.pl and put this text inside it:

##
# 
# pingentry.pl
#
# code snippet from Ben Trott on the MT support board
#	http://www.movabletype.org/support/
#
# wrapped up nice and neat like this by 
#	Bill Kearney
#	http://www.ideaspace.net/users/wkearney
##

use strict;
use MT::Template::Context;
use MT::Trackback;

MT::Template::Context->add_container_tag(PingEntry => sub {
   my($ctx, $args, $cond) = @_;
   my $p = $ctx->stash('ping')
       or return $ctx->_no_ping_error('MTPingEntry');
   require MT::Trackback;
   my $tb = MT::Trackback->load($p->tb_id);
   my $entry_id = $tb->entry_id || return '';
   require MT::Entry;
   my $entry = MT::Entry->load($entry_id)
       or return '';
   local $ctx->{__stash}{entry} = $entry;
   local $ctx->{current_timestamp} = $entry->created_on;
   $ctx->stash('builder')->build($ctx, $ctx->stash('tokens'), $cond);
}); 

Save the file and then upload it via FTP to your MT site 'plugins' directory. Once that's done your MT site will automagically recognize it. Check your web server error_log just in case.

The above plugin lets you use Entry tags inside an MTPings container. I've made use of is like so:

<MTPings lastn="15" sort_order="descend">			
  <h3 class="title"><a href="<$MTPingURL$>"><$MTPingTitle$></a></h3>
  <MTPingEntry><i>Re: <a href="<$MTEntryLink$>"><$MTEntryTitle$></a></i><br/></MTPingEntry>
  <$MTPingExcerpt$>
  <div class="posted">Pinged: <$MTPingDate language="en" format="%a, %d %b %Y %H:%M:%S"$> from: <$MTPingBlogName$></div>
</MTPings>