im pretty n00bish in all this, but if it is the forum button in the top menu you are after, i might have a fix!
i made mine with 2 steps : since the index.template calls for txt strings, i went to the corresponding language file (i think it was index.english" in default theme\language and added some txt strings. like this :
$txt[17] = 'Modify';
$txt[18] = 'Forum';
$txt[19] = 'Members';
$txt[20] = 'Board name';
$txt[21] = 'Posts';
$txt[22] = 'Last post';
$txt[998]='Media';
then i could easily go into template.index and do this (should be around line 425)
// Show the [home] button.
echo '<li><a' , $current_action=='home' ? ' class="current"' : '' , ' href="', $scripturl, '">' , $txt[103] , '</a></li>';
// Show the [help] button.
echo '<li><a' , $current_action=='forum' ? ' class="current"' : '' , ' href="', $scripturl, '?action=forum">' , $txt[18] , '</a></li>';
// How about the [search] button?
echo '<li><a' , $current_action=='media' ? ' class="current"' : '' , ' href="', $scripturl, '?action=media">' , $txt[998] , '</a></li>';
this dont actually add a new button tho, it just replaces the button text and their actions after your fitting