Hello,
I am using Infopath 2003.
Luckily, I also found a way round this.
I add the secondary data source to my script like this:
XDocument.DataObjects("Languages").DOM.setProperty("SelectionNamespaces", 'xmlns:prj="http://schemas.microsoft.com/office/infopath/2003/dataFormSolution"');
where Languages is a Sharepoint list,and then, when attempting to select the check box in order to set its value I use this code
var node = XDocument.DataObjects("Languages").DOM.selectSingleNode("//*[@Language_code='value']");
where value is a the value of the attribute bound to that checkbox.
I hope I made myself clear :)
Monica