If you don't want to display "Last post by" in private forums, use this code, other wise, use the one at the bottom:
in Ultimate.cgi find  
}  else {
$GetMonth = "";
$GetDate= "";
$GetYear = "";
$LatestTime = "";
$TheDate = "";
}

Right after that add 
 
if ($thisforuminfo[6] ne "private") {
@foruminfo = OpenFile("$ForumsPath/$ExactPath/forum$x.threads");
$size=@foruminfo;
$ThisForum="$ForumsPath/$ExactPath/forum$x.threads";
if ($size ne "0") {
$ThisLine=$foruminfo[($size-1)];
@ThisForum=split(/\|/,$ThisLine);
@ThisTread = &OpenFile("$ForumsPath/$ExactPath/$ThisForum[2].cgi");
$ThisThreadSize=@ThisTread;
$ThisLine=$ThisTread[($ThisThreadSize-1)];
@ThisThreadLines=split(/\|\|/,$ThisLine);
$LatestPostBy="by $ThisThreadLines[2]";
	}
else{
$LatestPostBy="&nbsp\;";
	}

}
else{
$LatestPostBy="&nbsp\;";
}

Now in each cycle of $GutsRow add this:
 
<br><b><FONT SIZE="-2" FACE="$FontFace">$LatestPostBy</FONT></b>

right after
 
<FONT COLOR="$TimeColor" SIZE="$DateSize" FACE="$FontFace">$LatestTime</FONT>
######################################################
To display in ALL forums:
in Ultimate.cgi find  
}  else {
$GetMonth = "";
$GetDate= "";
$GetYear = "";
$LatestTime = "";
$TheDate = "";
}

Right after that add 
 
@foruminfo = OpenFile("$ForumsPath/$ExactPath/forum$x.threads");
$size=@foruminfo;
$ThisForum="$ForumsPath/$ExactPath/forum$x.threads";
if ($size ne "0") {
$ThisLine=$foruminfo[($size-1)];
@ThisForum=split(/\|/,$ThisLine);
@ThisTread = &OpenFile("$ForumsPath/$ExactPath/$ThisForum[2].cgi");
$ThisThreadSize=@ThisTread;
$ThisLine=$ThisTread[($ThisThreadSize-1)];
@ThisThreadLines=split(/\|\|/,$ThisLine);
$LatestPostBy="by $ThisThreadLines[2]";
}
else{
$LatestPostBy="";
}

Now in each cycle of $GutsRow add this:
 
<br><b><FONT SIZE="-2" FACE="$FontFace">$LatestPostBy</FONT></b>

right after
 
<FONT COLOR="$TimeColor" SIZE="$DateSize" FACE="$FontFace">$LatestTime</FONT>

It was tested on 5.42 and 5.41. Should also work on previous versions, though!