You should contact the plugin author, because plugin support is generally off topic on this site.
But … I got curious, peeked into the plugin code and found this line:
add_action( 'wp_head', array('dc_jqverticalmegamenu', 'header') );
where the header() method is assumed static, but it’s not:
function header(){
// ...
}
That’s why this strict notice pops up.
The plugin creates an instance of the class through:
// Initialize the plugin.
$dcjqverticalmegamenu = new dc_jqverticalmegamenu();
so you should instead try:
add_action( 'wp_head', array( $this, 'header') );
and similar for the other static calls.