First of all, everyone here is volunteer who work on their time. This can be urgent for you but not for others. Don't ask questions like this one as it won't help you but response will be very less.
Further, change the way you think. You are not suppose to access page controls from other pages. You have to pass values of selected checkboxes to that page. So try searching on
How to: Pass Values Between ASP.NET Web Pages[
^]
Try something before posting something.