just a few steps to make run the module

1. 
copy the complete PostingSubtitle-folder into the mods-folder

2.
open the file templates/<your template folder>/posting_messageform.tpl, search for '{HOOK tpl_editor_after_subject}'. BEFORE this, put in:
	{IF mod_PostingSubTitle->enabled}
	<tr>
		<td style="white-space: nowrap">{LANG->mod_PostingSubtitle->capture}:&nbsp;</td>
		<td><input type="text" name="mod_PostingSubTitle_sub" id="mod_PostingSubTitle_sub" size="50" value="{mod_PostingSubTitle->value}" /></td>
	</tr>
	{/IF} 

3.
open the file templates/<your template folder>/list.tpl. Search for '<a href="{ROWS->url}">{ROWS->subject}</a>' AFTER this, put in:
	{IF mod_PostingSubTitle->enabled}
		<br />{ROWS->meta->mod_PostingSubtitle->subtitle}
	{/IF}

4.
open the file templates/<your template folder>/read.tpl. Search for 
	<div class="PhorumReadBodySubject">{MESSAGES->subject} <span class="PhorumNewFlag">{MESSAGES->new}</span></div> 
AFTER this, put in:
	{IF mod_PostingSubTitle->enabled}
	  {IF ROWS->meta->mod_PostingSubtitle->subtitle}
	      <br />{ROWS->meta->mod_PostingSubtitle->subtitle}
	  {/IF}
	{/IF}

5.
enjoy :)