I am having the same exact issue as mentioned here: http://social.msdn.microsoft.com/Forums/en/sharepointinfopath/thread/d83e7c6e-f01f-42c9-94fd-20ca78db27d8.
Here is my code: public void CTRL1_5_Clicked(object sender, ClickedEventArgs e)
{
string url = MainDataSource.CreateNavigator().SelectSingleNode("/my:myFields/my:field1", ...