Since Roku does not allow you to manually change your DNS settings you will have to configure your router in order to use SmartDNS with your Roku player.
In order to use some apps with Roku you also need to block access to 126.96.36.199 and 188.8.131.52 from the router. Please see this guide.
Getting US Channels on Your Roku From Outside the US
In order to add US apps to your Roku account you will need to have a Roku account located in the US. Simply selecting US as a country during signup will not be enough since Roku will check your location during the signup process.
We have a step based tutorial here as well: How to get US Channels on Roku
In order to get around this problem, you will first have to configure your router with SmartDNS. Then once your setup has been verified in your SmartDNS dashboard you can now visit http://owner.roku.com and create your account.
If you choose your credit card as your payment method, fill out all info with US info and once you have to input your zip-code simply make sure that the zip-code has 5 digits and that the last are included in your real zip-code. Lets say my real zip-code is 1600 I will then put 01600.
Now you have a US based Roku account and you can add the channels you need to your account. Then proceed to do a normal setup of the Roku box.
If you have already setup your Roku box with a non US account you will have to reset the Roku to factory settings.
From the Roku home screen, select settings > factory reset, and follow the on-screen instructions.
First step of troubleshooting should always be to restart device(s) and router. Then check your setup status in your SmartDNS dashboard.