I have crested one test application for your scenario and its working great, please have a look into the below example :-
Controls in my page :
i) A 'asp:DropDownCheckBoxes' control
Ex :-
<asp:dropdowncheckboxes id="ddllanguage" xmlns:asp="#unknown">
runat="server"
onchange="onSelectionChange()"
AddJQueryReference="True"
UseButtons="false"
UseSelectAllNode="True"
ClientIDMode="Static">
<style selectboxwidth="200" dropdownboxboxwidth="200" dropdownboxboxheight="130" />
<texts selectboxcaption="Select Values" />
<items>
<asp:listitem text="Test1" value="1"></asp:listitem>
<asp:listitem text="Test2" value="2"></asp:listitem>
<asp:listitem text="Test3" value="3"></asp:listitem>
<asp:listitem text="Test4" value="4"></asp:listitem>
</items>
</asp:dropdowncheckboxes>
ii) A textbox to display the selected values
Ex :-
<asp:textbox runat="server" id="txtValue" clientidmode="Static" xmlns:asp="#unknown"></asp:textbox>
Now the jQuery code to populate the selected checkbox texts into the textbox as comma separated values :-
function onSelectionChange() {
var selectedVals = '';
$.each($('#ddllanguage input[type=checkbox]:checked'), function () {
selectedVals += $("label[for='" + this.id + "']").html() + ",";
});
$('#txtValue').val(selectedVals);
}
NOTE : For getting selected values simply you can concat the 'this.value' to textbox rather than using the above snippet '$("label[for='" + this.id + "']").html()'
The jQuery version used for the example is 1.7.1, you can use accordingly.
Hope this will definitely give you hint how to proceed.