Hi Gastón,
First of all, I've found the Adobe XML Form Object Model Reference a great help in these scripting questions. You can get it here:
http://partners.adobe.com/public/developer/en/xml/Adobe_XML_Form_Object_Model_Reference.pdfWhat you're looking for is the addItem method. It's defined in the above document on page 581.
This isn't exact code, but just to give you an idea of where you need to go... let's say your country SOM-expression is xfa.form.Country and your state SOM-expression is xfa.form.State.
In the "exit" event on my xfa.form.Country field, I'd place an if-else statement.
if (xfa.form.Country == "United States") {
xfa.form.State.clearItems();
xfa.form.State.addItem("Alaska");
xfa.form.State.addItem("California");
}
else if (xfa.form.Country == "Mexico") {
xfa.form.State.clearItems();
xfa.form.State.addItem("Chihuahua");
xfa.form.State.addItem("Sonora");
}
else {}
Again, not tested code, but should give you a good idea of where to go.