You are creating a Custom Action, that does something in a SP modal dialog and you want to read whitch list items have been selected by the User then you must use this method:
SP.ListOperation.Selection.getSelectedItems
See the following example:
var clientContext = new SP.ClientContext.get_current();
var myWeb = clientContext.get_web()
clientContext.load(myWeb);
var selecteditems = SP.ListOperation.Selection.getSelectedItems(clientContext);
var item;
for (item in selectedItems)
{
//Whatever you want!
}
Bye
