First of all, you're using Acrobat JavaScript in an XFA form, which doesn't work.
LiveCycle Designer has its own interpretation of JavaScript, which is different to Acrobat.
There is also a second scripting language called FormCalc available.
Its syntax is much easier to learn and is matches perfectly to manipulate XFA forms.
Check the help (F1 button) in Designer to get the Designers scripting guide.