in answer the original question your page references were incorrect. you have to get a reference to the calling page from the Server.Request object. Cast that to a Page object then findcontrol for your tabs. This is better than javascript as it is all done on the server. There are plenty of articles on how to get the calling page.
The alternative is to use session objects etc as mentioned above. The best way is to pass the information in as a query so when you redirect from one page to the other grab the labels value and pass it to the next page in the request eg www.yoursite.com/newpage?labelvalue='the value you just grabbed'
This will guarantee the value will get there (no relying on cookies being turned on etc) and easy to get at the other end