Here is a routine to select set items and their check/uncheck checkboxes in Javascript.

Hope it helps someone like me who spent some good time trying to figure out why this wasn't as easy as in code behind.

I am sure there are more approaches, but this sub will get you started.

function SetValuesCountryofSales(paramitemstomark) {
            var witemstomark = paramitemstomark.split("~")
            var items = _WebDropDown.get_items();   /* Initilized before */
            var i = 0;
            var item = '';
            for (i = 0; i <= items.getLength() - 1; i++) {  /* Loops through the whole cbo */
                item = items.getItem(i);
                item.unselect();    /* this is if there where selected items */
                item.get_element().childNodes[0].checked = false; /* this is if there where selected checked */

                for (j = 0; j <= witemstomark.length - 1; j++) {
                    if (item.get_value() == witemstomark[j]) {
                        item.select();
                        item.get_element().childNodes[0].checked = true;
                    }
                }
            }
        }

Good luck,

Federico.