Getting selected data out of a ModalPopup

Now I'm thinking I may only have to write a quick client-side script that would be called via my TR's onClick event. The Repeater would render the TR's onClick to call the function along with the necessary data; the function would add that data to a ListBox back on the main form and then call the ModalPopUp's hide() method. Alas, I can't seem to get that to work either ... am I way off base here?