I suggest to use the RadTabStrip's OnClientTabSelected event instead of the OnClientTabSelecting - you can see how to hook it up here
Instead of the alert message you should open your window in one of these ways
(please, note, that if you use radopen you should have declared a RadWindowManager on the page).
As to the RadWindow's appearance, you should simply build the page you want to show in it (with the RadioButtonList, etc) and then set the RadWindow ContentUrl property to its url.
In case you experience problems, please open a new support ticket and send us a sample project which demonstrates the issue.
the Telerik team