I just answered your support ticket on the matter, yet for your convenience and for others that may experience a similar difficulty I will paste my advise here as well. For the sake of avoiding duplicate posts I would advise in the future that you open only one ticket per issue.
If I understand your issue correctly - you need to pass content from one page to another in the code-behind. This is considered general ASP knowledge and task and is not directly related to our controls. There are some ways to achieve such cross-page communication and it is usually via public properties and methods, you can examine the following links to get you started on this:
or the following google search on a similar matter:
All this being said I believe there is another possible approach you can take - you could try using the RadWindow's ContentTemplate to put the progress area controls in it. It does not require an external page and does not create an iframe, instead the content is a part of the main page. This way you can easily access the controls in the code-behind. For more information on the difference you can examine this online demo
and this help article
. The only issue with this approach is that this functionality is relatively new and it is not present in the version you are currently using. Therefore I would advise that you upgrade to the latest version, as thus you will also benefit from the new fixes and features that have been incorporated in the past three years.
If you are still experiencing difficulties I would also recommend that you examine the second option from the KB article that also keeps the progress area on the main page, just positions it absolutely.
the Telerik team