1. First go to Settings then select System
2. Select Network Settings
3. Select your network
4. Click on Configure Network
5. Select DNS Settings
6. Click on Manual
6. Then Select Primary DNS Server
7. Set it to 126.96.36.199
8. Then Select Secondary DNS Server
9. Set it to 188.8.131.52
10. Select Done
11. Verify your new DNS settings
12. Restart your Xbox
Remember you will need a US Xbox Live Gold account to install US apps on your Xbox 360. Please see this guide.
First step of troubleshooting should always be to restart device(s) and router. Then check your setup status in your SmartDNS dashboard.