MultiSelect is different than select on populating the date. for MultiSelect please find the example below :-
var selectElemet = document.createElement('select');
for (var i in gridData.items) {
var opData = document.createElement('option');
opData.innerHTML = gridData.items[i].description;
opData.value = gridData.items[i].value;
selectElemet.appendChild(opData);
}
var myMultiSelect = new dijit.form.MultiSelect({
name: c['srchDimnId'],
id : 'elementDimnSearchGrid' + dimnSearchIndex,
height: '200px'
}, selectElemet).startup();;
where u loop through the list and create an option for each of it, then append it to the select. make sure to pass the select element to the MultiSelect
please refer to the this link for details dojo MultiSelect