Well, it's not so clear what you are really asking for. There is no "multiple selection dropdownlist" in HTML. To have a multiselect you need to specify
<select id="foobar" multiple>
that will create a listbox where you may select multiple elements. Calling
var sel = $('#foobar').val();
will return an Array of selected items.
edit
To get the text from each option, use .map() or jQuery.map(). Example:
var sel = $('#foobar').children('option:selected').map(function(i,e){
return e.innerText;
}).get();
That will create an Array containing the text of all selected entrys.