Follow one of these methods to test popup web pages using WatiN.
//after the click that opens the popup:
IE iepopup_1 = IE.AttachToIE(Find.ByUrl(theUrlOfThePopup));
//operate on iepopup_1
IE poppedUpBrowser = IE.AttachTo(Find.ByUrl(“http://www.popped-up-site.co.uk/”));