I'm sure this is a common question. The best way to do this is using an advanced HTML theme:
http://www.question2answer.org/advanced.php#theme-advanced
Your theme's qa-theme.php should contain this:
<?php
class qa_html_theme extends qa_html_theme_base
{
function head_custom()
{
if ($this->template=='question') {
$this->output('<META NAME="description" CONTENT="'.strip_tags(@$this->content['q_view']['content']).'"/>');
$this->output('<META NAME="keywords" CONTENT="'.strip_tags(implode(', ', @$this->content['q_view']['q_tags'])).'"/>');
}
}
}
?>
What's happening here is that your theme is overriding the default head_custom() method (which does nothing) to output the meta tags, if this is a question page.
If you don't want to mess with advanced themes, you can also use the above function head_custom() to replace the one in qa-theme-base.php, but this will make it harder for you to upgrade to future versions of Q2A.
See also my comments here regarding whether meta tags are worthwhile:
http://www.question2answer.org/qa/1153/description-meta-keywords-front-page-then-related-questions#1163