Well.Try this:
1. Open forum/memberlist.php. Find
$pm_link = "href="{$pmplugin_link}">";
2. Replace with:
PHP Formatted Code
$pm_link = "href=\"{$_CONF['site_url']}/privmessages/compose.php?to_uid={$siteMembers['uid']}\">";
3. Open forum/include/gf_showtopic.php. Find
$pm_link = "href="{$pmplugin_link}">";
4. Replace with:
PHP Formatted Code
$pm_link = "href=\"{$_CONF['site_url']}/privmessages/compose.php?to_uid={$showtopic['uid']}\">";
I don't have new forum installed and
I don't tested this hack. I just looked into the code. Tell me if it doesn't work.