If you're using the Firefox browser to edit your website, you might have a problem to use a Matrix Theme. The bug won't allow you to click /select any element from admin menu.
There are 2 ways to solve the problem with admin menu:
1) If you're using a recent version of Matrix theme, just upload a new js file 'Matrix-1.9.3' that contains the Firefox bug fix
2) For the old templates using matrix 1.4, 1.6 or even 1.8, you can add the following code inside the Edit Head section:
<script type="text/javascript">
//<![CDATA[
jQuery.noConflict();
(function($) {
// Init
$(document).ready(function() {
// firefox bug fix for admin menu
$('.cd-btn').on('click', function(event){
event.preventDefault();
$("#cms", window.parent.document).parent().parent().parent().parent().parent().css("top", "0").css("height", "100%");
});
//clode the lateral panel
$('.cd-panel').on('click', function(event){
if( $(event.target).is('.cd-panel') || $(event.target).is('.cd-panel-close') ) {
$("#cms", window.parent.document).parent().parent().parent().parent().parent().css("top", "48px").css("height","calc(100vh - 48px)");
event.preventDefault();
}
});
});
})(jQuery);
//]]>
</script>