Deep breaths, eriasor.
It sounds like you got the first warnings when you were installing the component? Do you typically get warnings when you install components? I just installed it on a clean Joomla 1.5.9 installation running on Windows with display_errors on and didn't see any warnings. The code that it is referring to (simplexml.php) is core Joomla code. We could definitely be causing the warning, but I wouldn't have expected it to install the component at all if that were breaking.
This is line 44 of usermap.php:
$this->_data->created_at = JFactory::getDate()->toMySQL();
Not sure why this wouldn't be working, but we might find out after you give us some more info.
What version of Joomla are you using?
Windows or Linux?
Apache (or IIS) version?
PHP version?
MySQL version?
We'll get you working. You just keep taking deep breaths
Will