That should work as far as I can see. The default is:
<meta name="description" content="<%meta_name%>">
<%endif%>
<%if meta_keywords%>
<meta name="keywords" content="<%meta_keywords%>">
<%endif%>
Are you sure meta_name is defined for that category?
Code:
<%if meta_name%> <meta name="description" content="<%meta_name%>">
<%endif%>
<%if meta_keywords%>
<meta name="keywords" content="<%meta_keywords%>">
<%endif%>
Are you sure meta_name is defined for that category?