First thing you want to do is add fields to your category table, so from your main admin page, Tools, then click on Category on the left. Add text fields for each meta tag you want to add. You can then add the data to each category via Forums at the top of your admin pages.

Then edit category_view.html to add in the tags, something like:

<%if cat_description%>
<meta name="description" content="<%cat_description%>">
<%endif%>

You could do something similar for individual forums as well, in which case you'd want to add the fields to the forum table and modify forum_view.html