Again, I am wondering why you are using code for this.
However, if you are sure your code is not the issue, drag the my:myFields/my:Countries field onto your form as a repeating table and preview your form. Do you see the data in that table that you'd expect? If so, the issue is with your drop down -- make sure you select the option that the data for your drop down is in your form, and select the Countries field. If the data is NOT there, the issue is somewhere in your code.
If your issue is that your code works in client, but not in browser, you probably need to admin approve your template for use in the browser since it has managed code.