Placing Ads after every 10th Post with Infinite Scroll

See this: http://infiniteajaxscroll.com/docs/events.html#rendered

So, place all the ad code in an ajax function (http://codex.wordpress.org/Plugin_API/Action_Reference/wp_ajax_%28action%29)

add_action( 'wp_ajax_your_ad_fn', 'your_ad_fn');
add_action( 'wp_ajax_nopriv_your_ad_fn', 'your_ad_fn');

function your_ad_fn(){
    // your ad code here
    // if sending js, set Content-type header to application/javascript
    // if sending html, text/html
}

https://stackoverflow.com/questions/9664282/difference-between-application-x-javascript-and-text-javascript-content-types

Then in your custom js,

ias.on('rendered', function(items) {
    var $items = $(items);
    jQuery.get(
        'http://yoursite.com/wp-admin/admin-ajax.php', // I've just illustrated, don't write the url directly, do it via wp_localize_script
    ).done( function( data ) ){
         $items.after(data); // append ad after all the loaded posts
    });        
})

This is NOT the exact code, you have to work out some details, yourself.