Did you create this theme by yourself or is it a free/premium theme bought somehwere?
This way it’s hard to say what might be the issue with the plugin. But to be true, I’m shocked that this site works at all.
Just take a look at any of the default WP themes archive.php file. That’s the best source to look for. You should also get familliar with the WordPress template hierarchy (http://codex.wordpress.org/Template_Hierarchy) It will give you a general idea of what template file and when is being called.