If you want to preview branching logic and get the Congrats Page, you will have to make sure and enter all of the correct information into your Ethnio screener, based on the logic that you have in the "Who gets an invite?" section on the Thanks page.
Make sure that you fill out the name and e-mail fields, as Ethnio will cleverly not show the Congrats version of the thanks page to anyone that leaves blank fields in their screener response. It's a way that we guard against speedy fakers trying to participate in your research. Keep in mind that you should be using a unique e-mail each time you preview the branching logic and want to see the Congrats page.