Unfortunately, this was moved to include/func/func.core.php class XCAccountTabs. You'll see similar tab assignments there.
Rather than editing the core, a slightly less invasive way is to edit postauth.php by finding this line:
PHP Code:
XCAccountTabs::getInstance()->addDefaults()->assignToSmarty();
And add something like this after:
PHP Code:
$myNewTab = array(
'title' => 'lbl_downloads',
'url' => 'my_page.php'
);
$myAccountTabs = XCAccountTabs::getInstance()->addDefaults();
$myAccountTabs->push($myNewTab);
$myAccountTabs->assignToSmarty();
This expects a language variable (ex: lbl_downloads).